Injecting Water into an Anthropogenically Altered Groundwater System

Coal seam vector data was provided to the University in June 2016 by the Coal Authority who have digitised the principle mineworkings from the Brockwell seam up to High Main (see Fig. 5). This data was made possible via the 1872 ‘Coal Mines Regulation Act’ where mine owners were legally required to provide detailed sub-surface mine plans. Original mine plans were in two formats; 1) vertical cross sections highlighting depth and lateral extent of mineworkings and 2) Horizontal seam maps which provided quantities of coal removed that can be treated as an estimate of voidage at the time of surveying. A summary of the Coal Authority GIS data is shown below in Table 2.

A drawback of the Coal Authority coal seam data is that it provides an overall areal blanket of ‘possible’ workings without the vital gallery, shaft, adit etc network data. Additional contact was made with the CA, however, these open void networks are yet to be digitised. Further discussions held at the Mining Institute in Newcastle proved fruitless due to bygone practices and general disarray of seam maps. Eventually the projects initial scope of digitising individual open-void pathways had to be scaled back due to the shear time involved in mineworking digitisation. Coal Authority data was first matched with the corresponding local seam name and then filtered for those workings directly within the research boundary, leaving 11 workings (Table 3)

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The 100-year CityCAT Newcastle city flood simulation was eventually obtained from a fellow student at Newcastle University. No editing was required and when integrated into ArcMap areas of surface ponding could be estimated as potential volumes to be stored in the sub-surface.

Rainfall data for the city was acquired from the MetOffice website.  MODFLOW models require a recharge value calculated from rainfall data to estimate recharge for the study area and thus calculating a water balance.

Groundwater levels were acquired from the Environment Agency for the period March 2004 – Feb 2016 using the five closest boreholes to the project area. Their details and locations are highlighted below in Table 4 and figure 25 respectively. Water level data will allow the confirmation of water table stabilisation (after pumping cessation), and if groundwater is still rebounding then allow evaluation of future rises. This value will assist in estimating the available unflooded mineworkings and will be the initial test of the concepts viability, i.e. Is the available voidage greater than surface flood water volumes to be stored. (Note: Requests were made direct to the EA and CA for up to date groundwater level data as well as for additional borehole points in the NW of the area, regrettably this data was not available).

3.2 Conceptualisation of the Model

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The external boundary of the study area was selected based on its surface and sub-surface properties. The River Tyne and the Ouseburn make up the southern and eastern boundaries respectively, with the 90-fathom fault dissecting NE-SW between the water waterways (Fig, 4). The latter fault boundary is suggested by Younger et al., (2012) to have some hydraulic connection from depth, convecting heated groundwater upward from the basement Weardale Granite, however, due to the model’s relatively shallow depth it is considered a no-flow boundary. The total conceptual area encompasses 27.2 km², while the depth of the model will vary between 115.7 in the west and 251.7 m in the east exploiting an argillaceous layer (Victoria Marine Band) beneath the Brockwell workings as a base.

Vertically, the study area holds 21 known worked seams (in addition to ‘probable workings’), each separated by intricate cyclotherm lithologies and a spattering of perched aquifers, thus this complex geology required simplification (Fig. 26). The digitised Coal Authority seam data of the primary mineworkings within the study area are (from lowest to highest working); Brockwell, Three Quarter, Bottom Busty, Top Busty, Tilley, Hodge, Harvey, Hutton, Brassthill, Lister and High Main. Thickness of workings were obtained from both the Durham Mining Museum Website and Audley et al., (1998), applying the thinnest seam working values to not overestimate the available voidage. These worked seams will be the principle route for groundwater flow within the model, exhibiting turbulent conductivities analogous to karst systems (Sammarco, 1995; Burbey et al., 2000; Cheney, 2007).

Lithologies between coal workings vary from medium-grained sandstone perched aquifers to low permeable muds, silts and seat-earths, and have been amalgamated into single permeable mediums, a crude but necessary method of capturing the complexity of cyclotherm geology. As strata shallowly dips 3-5° ESE, it is suggested by the CA that the regional groundwater flow follows a similar route towards the Tyne and North Sea, however, the complex anthropogenically altered geology may direct flow towards the Tyne or/and Ouseburn, a subject of interest to this project and explored in section 3.4.1.

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Recharge will occur via effective rainfall calculated from data obtained from the Met Office (~ 655.5 mm/a). Due to urbanisation of the city, it is predicted that when utilising an urban surface blanket as the top layer of the model, this will result in recharge primarily occurring in the rural (uncovered) north-west and Town Moor localities.

It should be expected that an injection of water into a hydrogeological system will create a groundwater mound that propagates from the injection well towards the head boundaries of the rivers. The workings submerged beneath the known water table will have the major control over a ‘pulse’ of injected groundwater flow. Therefore, attention should be drawn to the lower workings that intersect the banks of the River Tyne in the South-East of the study area. Groundwater flow direction may be determined by comparative analysis of hydraulic heads between the water table and the River Tyne, where a higher water level in the river means that the Tyne is leaking into the subsurface, and vice versa.

Within submerged mineworkings, three types of none-Darcian flow exist; laminar (turbulent

Poiseuille flow), turbulent convective flow and Brownian Diffusion (Wolkdersdorfer, 2006). These flows occur in tandem with one another and often change flow regime within a short distance. Therefore, describing flow within a flooded mine system is extremely difficult, more so when considering flows within the void walls, the solid coals pores and the surrounding fractures caused by excavation.

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The flood water injection location, may have influence on localised groundwater flooding due to the interconnectivity of different workings. Water input in the east would most certainly convey groundwater level rises to the west as water fills empty strata.

3.2.1 Recharge

De Vries and Simmers (2002) define groundwater recharge as “The downward flow of water reaching the water table, forming an addition to the groundwater reservoir”. Although grossly oversimplified, the process must account for evapotranspiration, the soil moisture deficit and impermeable urbanised surfaces. Evapotranspiration is the combination of ‘evaporation’; the upward movement of moisture into the atmosphere and ‘transpiration’; the absorption of water through vegetation. The soil moisture deficit is the amount of rainfall required to return the soils moisture content back to ‘field capacity’. Once a soil is saturated, it will no longer retain water within its pores through capillary action and allow water to drain freely downward.

Newcastle upon Tyne exhibits two main surface coverings (urban and rural) which cap superficial glacial tills that cause recharge fluctuations to the confining the minor aquifers below (Simpkins & Bradbury, 1991). Therefore, to calculate effective recharge to the MODFLOW model a recharge retardation factor (Fig. 27) can be applied to Turc’s recharge calculation (Eq. 1).

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The 100-year flood CityCAT and JFLOW model outputs of Newcastle city centre were analysed with regards to surface flow direction and ponding location. This will highlight high risk flood zones (Fig. 28) and provide an estimated volume of water to be subsequently linked to the nearest mine entrance via multi criteria analysis (MCA). Masser & Jensen (1991) provide a basic methodology on pond volume estimation using surface area * average depth. ArcMap 10.3™ will be utilised to measure areas of surface ponding and when combined with a DTM and the flood models’ depth data, estimate potential volumes to be stored. Furthermore, suitable mine entrances will be chosen via MCA using a weighting system based upon the following factors; Mine entrance angle (excluding adits) 0.2, proximity to mine entrance 0.6, depth to workings 0.1 and the hypothesised available voidage 0.1.

3.3.1 Issues with Urban Flood Models

Initial troubles in obtaining the 100-year flood CityCAT output led to disruption to the proposed project timetable. Requests were also made to Paul Wass of JBA for a comparative JFLOW output of the flooding of Newcastle. In the end, both models provided valuable insight into the events of 28^th of June 2012, allowing estimations of flood waters to be gathered.  The assumption was then made that flood waters are to be induced into the mineworkings via hypothetical boreholes sunk vertically into the nearest mine entrance or working, as unfortunately the engineering behind this methodology is beyond the scope of this project.

3.4 Voidage Calculation (ArcMap & ArcScene)

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3.4.1 Working Interconnectivity & Refinement

The raw Coal Authority GIS files were firstly refined within ArcMap and clipped to the models conceptualized boundary. Due to the lack of tunnel data, two simplified models with differing connectivity’s are proposed;

• A crude ‘blanket’ of workings that is hypothetically wholly connected utilizing a common set of hydraulic properties, where coal measures are linked by hypothesized vertical shafts where indicated in the literature.
• A sophisticated pond system which divides up each coal measure by known unmined strata acting as barriers to flow. Note: A guide to building linked ponds within coal mines is provided by Younger & Adams (1999).

To enable each coal workings to lay upon its respective stratigraphic surface, TIN’s (Triangular Irregular Network) were constructed using seam contour and point data. The corresponding mineworking was then laid upon each TIN using the ‘base height’ command.

Inspection of the ArcScene model as well as the literature allowed assessment of void linkages between workings. These observations expanded upon the exceptional work already provided by Crawley (2016) and seek to further examine the dip and outcrop of the seam vector data. Seams that contour within 25 m of the DTM surface are deemed probable ‘seepage faces’ for groundwater flooding, especially those below the existing water table.

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3.4.2 Total Voidage Estimation

Each clipped mineworking extent was then extrapolated into a volume by multiplying the smallest working thickness provided in Audley et al., (1998) and where not present the Durham Mining Museum Website (Table 5 and previously in figure 27).

thinnest working height found in the literature was used in order to estimate the smallest available voidage, thus attempting to alleviate lost voids caused by unknowns such as collapse and backfill which is not well documented in the mining records. The initial value is of course a gross overestimate of the available space, as it includes the entire workings areal extent and ignores the ‘Pillars’ of the pillar and stall mining methodology.  Knowing that the majority of workings are ‘pillar and stall’ beneath Newcastle (WYG, 2011) the value was multiplied by a conversion factor of 0.25, 25% being the average amount of coal removed using this method (Atkinson, 1966). This again is a value that will predict the smallest available voidage within the workings, Younger and Adams (1999) suggest a storage coefficient of coal workings to be ~30%.

Finally, the available storage volume can be calculated by subtracting the voidage currently beneath the water table. This was done via the construction of a TIN within ArcScene using borehole water level data provided by the EA and then removing the voids below the water table. The 3D analyst tool ‘add Z information’ allows the selection of the Volume parameter which then calculates the remaining voidage.

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3.4.3 Issues with CA Data & Voidage Calculation

It was assumed at the beginning of the project that the data provided by the CA was a network of mining excavations giving a ‘true’ volume of voids, as well as their flow pathways. The workaround taken by the writer to portray the workings as a singular value with an average porosity leads to two major issues. Firstly, as Newcastle was host to a number of mining techniques, all of which excavated various percentages of the coal seams, the voidage estimation is prone to error. This error is enlarged by undocumented volumes of backfill and natural collapse, as well as the additional secondary porosity which is created within the weakened mineworkings walls (Younger, 2016). Of greater importance, with no linear network data available, the use of the CLN extension within GMS to model discrete outflow points becomes impossible and refinement of the mining network would be required to complete the task.

Time consuming methods to adapt the CA vector data to the CLN process included the introduction of a ‘pillar and stall’ gridded network into each seam layer, in an attempt to simulate the open channels found in mine networks. This unfortunately proved fruitless and an attempted output is presented in

The predominantly dry workings within the study area are those in the High Main seam, this poses a problem as the majority of these workings reside to the east of the city and well outside of the study area. There is a vast unknown voidage held within the ‘probable workings’ vector data which unfortunately varies in both depth (0-100+ m bgl) and method of working, and therefore should not be considered for voidage.

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Mineworking linkage vector data from the CA is sparse and is no doubt an underestimate of the systems true interconnectivity. Common mining practice was to backfill exhausted sections of the mine with waste material or ‘goaf’. Younger and Adams (1999) suggest this to have a hydraulic conductivity similar to gravel and a porosity of 0.3. Shaft data is also lacking in direct connections between seams and the surface, therefore for the purpose of the model, hypothetical injection wells will be added to direct flood waters downward.

Visits to the Mining Institute in Newcastle allowed inspection of old seam maps, which when compared to the vector data provided by the CA has some discrepancies; including; the absence of digitised Low Main seams which are clearly within the study region seam maps and missing roadways/linkages. These could be due to a number of factors including, processing time, nomenclature issues or simply that they are yet to be digitised.

3.5 MODFLOW-USG

The purpose of producing a numerical model is to confirm the conceptualisation, highlight any conflicts, analyse groundwater flow direction and simulate future scenarios beneath Newcastle. MODFLOW-USG code was selected to model the study area because it remains a relatively unexplored methodology for the modelling of mineworkings while exhibiting a number of desirable attributes which lend themselves to the complex hydrostratigraphy of the region, including;

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• USG allows pinching out of complex strata that does not span the model areal extent. Layers are no longer required to be laterally continuous, providing a more realistic representation of complex systems, while avoiding common issues with model convergence due to diverse hydraulic values.
• Unstructured grids allow increased refinement around a point of interest while reducing cell resolution in those areas of less importance – allowing a ~70% reduction in total cells.
• Cell reduction is also gained from a reduced domain size which in structured grids contain cells outside the modelled area.
• Up to a 90% reduction in simulation run times (Waterloo Hydrogeologic, 2016) – related to a reduced number of grid cells which are rendered each simulation.
• The UGrid cell geometries include voronoi polygons which accommodates complex geologies while providing greater accuracy and better model convergence than structured grids.
• CLN add-on module is specifically designed to simulate turbulent flows within groundwater pipe networks. This is something still in its infancy that has so far only been tested within karst systems.

3.5.1 Proposed Setup of MODFLOW Model

The model creation followed tutorials by GMS creators Aquaveo at http://www.aquaveo.com/software/gms-learning-tutorials.  This provided the basics for setting up a numerical model using the conceptual approach. A new conceptual coverage was added, creating a conceptual boundary and named ‘boundary’. This coverage is then duplicated 4 times to produce coverages specific to; Sources & Sinks, Recharge, Hydraulic Conductivity and Layer Elevation (Fig. 30).

The Sources & Sinks coverage defines local sources/sinks within the study region and has the options Wells, Refinement, Spec. Head (CHD) and Drain enabled, as well as turning on ‘Use to define model boundary (active area). Using the ‘Select Arcs’ tool, both the River Tyne and the Ouseburn arcs are classified as specific heads (CHD) and the 90-fathom fault was left as a no-flow boundary. The nodes at the end of each arc are each given their own unique head-stage, utilising values above sea level accessed from the OS database for each respective point. Finally, within the coverage, 3 proposed wells were selected based on their proximity to a mineworking known to be above the water table. They are situated at:

• Above the High Main workings, near South Gosforth (NZ25146799)
• Above the Tilley and Brassthill workings, near Benwell Football Centre (NZ206645)
• Above the Brassthill workings, near Manors station (NZ252642)

The model was proposed to be calibrated to standard conditions (no injection of flood water), then simulated for moderate inflow of surface waters (1500 m/s), followed by rapid influx of storm waters (5000 m/s). Surface water volumes are taken from the largest calculated pond (Civic Centre).

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• (At the end of each coverage creation the ‘Build Polygons’ function is pressed which rapidly defines all polygons within that coverage by searching through the arcs, creating a separate polygon where a closed loop is found).
• A recharge coverage was added to using Turc’s effective recharge value from equation 3. 282 mm/a was converted into 0.0077 m/d and assigned to the entire polygonal region. Hydraulic Conductivity coverage was set-up using a rural/urban surface polygon divide. Rural areas having a horizontal conductivity of 3 m/d and urban zones 0.1 m/d. It is noted that this is a crude method of dividing up such a diverse surface. Finally, an elevation coverage was added, utilising the Top and Bottom elevations within the Areal Properties. The maximum and minimum heights were 126.1 and 0 m asl respectively.
• The proposed model was to run as a pseudo-transient simulation, adding steps of increasing flood water inputs. Particle tracking will be utilised to predict groundwater flow direction and when cross referenced with outcrop geology estimate any seepage points. Analysis of the Coal Authority vector data uncovered that the seam working files were not suitable for integration with the CLN add-on module, attempts were made to introduce networks of ‘pillar and stall’ workings into each vector file without success. Initially the model was to contain 8 known working layers, contained by generic coal measures, argillaceous aquitards and a superficial covering (Fig. 26), however, due to raw data issues the model had to be refined as below.

3.5.2 Data Issues

• Early modelling issues arose with the vector data being incompatible with the GMS software, requiring a long-winded conversion process to raster’s, then to points, back to raster format and then into a TIN (thanks to Saul Montoya for this work around). These TIN files were then input into the raster calculator within ArcMap and extruded to their respective working depths.
• Further issues ensued from the placement of geological seams at their respective depths, often crashing the GMS software (Fig. 31). This same error occurred on university PC’s, home computer and via remotely accessed GMS software, suggesting that the data may have required refinement to reduce memory draw, something beyond the ability of the writer.
• Discussions held with MODFLOW ‘power users’ suggested the most common way of building a 3D geology within GMS is via digitising borehole logs. Attempts were made to construct boreholes from available BGS borehole scans, yet, unlike the tutorials, these home-made files did not transfer into GMS.
• Primary bottle necks during the modelling process were the type, quality and lack of data available. Methodologies currently exist for the construction of complex 3D stratigraphy within GMS (Panday et al., 2013; USGS, 2016; Herch, 2009, & Aquaveo), which provide step by step guides using borehole data. Unfortunately, boreholes would need to be individually generated from scans, which was trialled and deemed too time consuming.

3.5.3 Refinement of Modelling Techniques

• As previously mentioned the available vector data was not suitable for use with the CLN MODFLOW process as it lacked linear network cells to be designated an open pipe. On attempting to ‘convert to CLN process’ the option would remain ‘greyed-out’ which confirmed the lack of network data within the shape file. Research into the alternative turbulent flow modelling approaches underlined the practicality of the MODFLOW-2005 CFP (Conduit Flow Process) add-on. Again a methodology tried and tested within karst networks, where the process incorporates both none-Darcian flows and a diverse porosity of karst geology. The module utilises the Hagen-Poiseuille equation in laminar flow situations and the Darcy-Weisbach equation to simulate turbulent flow within open conduit networks (Hill, et al., 2010). The methodology benefits our predicament of a lack of network data, by allowing the blanket of workings to be assigned both hydraulic properties and a percentage of ‘open channel availability’ within the layer. Nevertheless, the process excels with a known pipe network input.
• With the placement of coal working TIN’s on their respective surfaces, the plan was to use ‘Horizons to solids’ to construct the 3D model, however, GMS would only occasionally provide a suitable output between TIN’s, often producing error messages, crashes, or anomalous shapes which did not resemble the proposed geological layer. The ‘Horizons to Solid’ methodology is also achievable via borehole data (unavailable) and raster catalogues (created during the TIN refinement process). Again, the resulting 3D solids were inconsistent and the methodology had to be rejected. A final attempt to create the hydrogeological strata within GMS was attempted by converting the TIN’s to a 3D mesh, regrettably this also failed instantaneously with errors (Fig. 32). (Note: a series of model versions with their various edits are located in Appendix A).

Due to a multitude of issues experienced with data, the writers lacking of experience with the GMS software and workarounds to proposed methodologies within the literature failing, it is proposed that future modelling attempts of the Newcastle Hydrogeology require the following:

• Digitised mine working network data (CLN or CFP processes)
• Borehole geological data set for the study region
• Additional training in 3D grid creation – Greater access to external resources
• USG / Ugrid workshops

3.5.4 Calibration of Simplified Lumped Model

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Due to a combination of the limitations of the writers MODFLOW skills, time and data availability, a lumped model was produced within GMS to endeavour to evaluate possible groundwater flow direction.

Layers which would not convert to GMS format were lumped with the closest coal working vector based on lateral extent, depth and dip of strata. Lumped coal working layers are as follows:

• Brockwell & 3 Quarters – Single Working, thickness combined to 1.9 m
• Busty’s & Tilley – Single Working, thickness combined to 1.9 m
• Hodge & Harvey – Single Working, thickness combined to 1.0 m
• Lister – Ignored due to small extent within the study area as well as erroneous vector data (Fig. 33).
• Probable workings are discounted due to issues with the vector data, combined with the unknown type and extent of the workings.
• Due to unsuccessful attempts to implement seam workings, coal workings are simplified to sheets connected to workings above and below via shafts.
• Removal of argillaceous layers – reduced complexity

4.  Results

4.1 Flood Water Volume (JFLOW Output)

Although a crude methodology was employed to tease out expected volumes of surface waters, the ponds causing the most havok within the city centre provide acceptable injection values for proof of concept during a storm scenario. Ponds range up to 3 meters deep (Manors, Civic Centre and Victoria Sq.) providing a city centre pond total of 141,190 m³ to be stored.

As this project does not investigate the engineering of the injection process, the ponds do not need to lie directly over a mine entrance, hence the Multi Criteria Analysis output below (Fog. 34) is simply a test of concept, with average proximity to known vertical entrances ~100.3 m.

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As can be seen from the voidage calculations (Table 7) the Brockwell seam working is the most widely excavated within the study area. Due to issues with the importing of the Hodge and Busty seams, these were then amalgamated in with workings of closest extent and dip. This also allowed the MODFLOW model to be simplified. Four sets of workings lie above the expected water table as of Feb-2016 (Fig. 35); High Main, Lister, Brassthill and Hutton, of which Brassthill provides over 62% of the available dry voidage. The ‘probable workings’ which the CA deemed ‘shallow’ (0-30 m bgl) were also superimposed onto the ArcScene providing an excess 12.9 million m³ of additional storage. These probable workings came from mines that predate 1872, before mine operators were legally required to lodge their mining plans. (Note: These values should be re-run once up to date water levels are obtained to confirm groundwater stability which is currently postulated to have fully rebounded WYG, 2011). There is a notable hydraulic gradient of 1.58 m (N-S) observed between boreholes at Freeman’s Road and Tyneside House. However, when considering values from the St. Anthony’s monitoring borehole in the south east of the region, water levels often exhibit a higher groundwater levels than expected, suggesting a south>north gradient, something which could be due to the influence of the open mine conduits.

Using a combination of the Mining Institute maps, coal mineworking hydrology literature, prior Masters’ studies of Newcastle workings (Crawley, 2016) and vector data provided by the CA, the following observations were made about the Newcastle Coal Mine System:

Linked Workings & Mine Entrances:

• High Main – Connected tentatively via vertical shafts to lowest seam in model (Brockwell) (Source: Mining Institute). Possible outcrops (Fig. 36) along the banks of the northern Ouseburn (Source: CA Vector Data). (Note: The high main most likely has a greater coverage across the western side of the study area, however, due to issues with the vector data is limited to the north eastern portion of the map).

• Lister – Top of vertically linked cluster of all lower coal workings (Source: MI). Minor outcrops in Scotswood (Source: CA).
• Brassthill – Primary outcrop along banks of River Tyne (Source: CA & BGS). Number of linked adits found along river banks, possible exit points for injection plume. Delavel mine adit is one such point (Fig. 37).
• Hutton – Workings disconnected from other seams within study region. Single outcrop in south east of study area with potential for seepage into Tyne (Source: CA).
• Harvey / Tilley / Busty / 3 Quarter / Brockwell – All largely worked across the study area and interconnected at a number of shafts (Source: MI & CA).
•  “Probable” shallow workings of depths less than 30 m below the surface litter the southern boundary with the Tyne (Source: CA). The extent, type of working and level of backfill are unknown making estimations of flow paths impossible. High chances of these workings being interconnected and therefore leading to surface seepage.
• Roadways (connections between worked seams) were previously examined by Crawley (2016) and suggest that although each seam is often divided into 4-5 worked areas, these workings remain well connected.
• All strata dip gently to the south east and includes three dividing aquitards.
• Seventeen documented adit mine entrances are noted within 250m of the watercourses, congregating along the Tyne from Scotswood to Benwell.
• 2.6 km of potential working outcrop along the banks of the Tyne and Ouseburn.

4.3 Groundwater Model (MODFLOW)

• Unfortunately, after numerous model attempts and gross simplifications on the initial concept a viable working GMS MODFLOW model could not be created with the data provided within the timescale available (Fig. 38). Conversations with the various MODFLOW communities have highlighted a number issues with the data and made recommendations for future study (Section 6.1). Discussions with Saul Montoya (HatariLabs) and Scott Paulinski (USGS) towards the end of this project have drawn attention to the overall scope of the intended project, underlining the need for additional data (borehole geology), common pitfalls in groundwater modelling and additional skills required to tailor raw data to our needs (e.g. Python). Scott was keen to call attention to the latest iteration of the MODFLOW code (MODFLOW 6) which is “more human friendly” than USG.

5.0  Discussion

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• The extensively intricate hydrogeology beneath Newcastle is made further complex by the addition of an estimated ~27 million m³ of mine voids. These interconnected systems obviously have a significant effect upon the natural groundwater regime, with flows preferring open shafts, galleries and roadways to the low permeable surrounding strata. Therefore, any groundwater model would struggle to model this system successfully, either by simplification of the system or conversely overparameterization of the study area.

5.1 Conceptualisation of an Anthropogenically Altered System

Groundwater flow tends to follow geological strata and preferential pathways such as open networks. The data provided by the CA only contains 30 horizontal connections between workings which may be a gross understatement. A common historical mining practice was to backfill a mined area with ‘goaf’ (waste material). This backfill has a high permeability not too dissimilar to gravel when uncompact and would act as additional connections between the workings. With regards to vertical mine shafts, groundwater flow will favour this open network over the surrounding low-permeable geology, however, the CA data suffers from a lack of depth data and therefore interconnectivity between layers can only be hypothesised (Fig. 39). As identified by Crawley (2016), information on vertical connectivity is only available for the major collieries, of which 4 reside within the study zone, out of a total of 283 mine entrances. This again is a coarse underestimation of the vertical connectivity within the conceptualised system.

When flow is impeded by strata of low permeability such as the argillaceous layers found beneath Newcastle, flow will begin to move horizontally. This horizontal flow can be postulated to flow under gravity along strata, and hence towards the south east of the study region, outcropping along the banks of the River Tyne and Ouseburn from seepage faces (Fig. 40) or mine entrances.

Non-Darcian flow rates will be greatest within the seams which have witnessed the largest percentage of extraction (High Main, and ‘probable’ workings), and those with the greatest extent (Brockwell, 3 Quarter, Tilley, Busty and Brassthill). Focus should be drawn to the lack of knowledge of the ‘probable’ workings beneath study area, of which 12.9 million m³ potential voids are located within the top 30 m of the subsurface. As these seams are suggested to have been worked extensively, exhibiting a wide range of mining methodologies it is extremely difficult to predict groundwater flow within this upper system.

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Unfortunately, residence and resurgence times could not be estimated without an accurate numerical model. Literature suggests that times depend on XXXX, and that follow will be dictated by the injection rate, dip of strata (~3 degrees ESE) and the mine network.

The proposed injection point were on the eastern side of the city, meaning that the strata is deeper and that as water is added flow moves to the west of the city.

5.2 Interpretation of MODFLOW Model

Due to the scope of the project’s concept, numerous data issues and the writer’s relative inexperience with GMS, a numerical MODFLOW model to complement the conceptualisation of Newcastle’s mineworking hydrology could not be created. Data integration difficulties included; extrusion of TIN layers within GMS (Fig. 41), implementation of raw vector data (Fig. 42), merging of raster layers into a single block layer (Fig. 43). However, it is of the writer’s opinion that the foiled attempts to apply the available CA data to a numerical model not go in vain. The rest of this section will highlight the proposed procedure to utilise both the CLN and CFP add-ons and highlight any additional data requirements.

The CLN add-on was the primary focus during preliminary investigations of suitable mineworking modelling processes. The add-on benefits from an unstructured grid which would potentially allow complex geology such as pinch outs to be modelled accurately and rapidly. Its considerable reduction in cell usage would also lend itself to the size and detail of the study zone. The basic modelling process is lain out below (Fig. 44) and required data in Table 8. Alternatively, the CFP expansion for MODFLOW-2005 has been successfully utilised to model karst networks and where available, also benefits from using linear pathways. Step-by-step CLN and CFP guides are available through the USGS, Aquaveo and other external sources (Table. 9), however, they are not aimed at beginners. requiring additional experience before attempting such a task.

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5.3  Potential Geohazards

5.3.1 Subsidence and Degassing

As previously discussed in the literature review, when groundwater rebounds due to an injection of flood water (or cessation of pumping), this will lead to a temporary rise of the water table. Initially a groundwater rise reduces the available ‘dry’ voidage and leads to upward migration of mine gases which become compacted between the confining surfaces, increasing the risk of land instability, explosions and poisoning from toxic gas accumulation. To pinpoint areas most at risk both the 3D ArcScene model along with Minining Institute maps were scrutinised. Workings outcropping along the banks of the Tyne and Ouseburn most at risk include;

• High Main workings along the northern Ouseburn (Fig. 45)
• Brassthill and Hutton Workings along banks of the River Tyne at Benwell and Scotswood (Fig. 46)

The Coal Authority state that pillar and stall workings often experience subsidence at the junctions of adit and shaft entrances, as well as roadways which tend to be closer to the surface. Further stability issues arise in the form of rock slope failure, which increases as water content increases (Turner, 1993). The risk of these two stability problems would certainly increase along the banks of the Tyne where multiple adits, shafts and coal seam outcrops are situated. The High Main coal seams near the north-east of the study area, are suggested to have had 90% of their coal extracted which are prone to collapse due to a fractured sandstone roof (Richardson, 1983). Frequent flooding/flushing of these workings could weaken these sandstone roofs via weathering or compression of gases due to reduction of space.

5.3.2 AMD

Known mine water outlets include the Delavel drift mine entrance (Fig. 47) which can be hypothesized as a potential surface discharge point for AMD triggered by a pulse of injected flood water. As acidic waters discharge a few meters from the River Tyne it would instantaneously oxidise producing a characteristic orangey brown ochre, covering the nearby banks. As well as being unattractive, AMD alters the current chemical conditions around the point of entry, gradually diffusing as it dilutes within the Tyne. AMD has a two-fold effect upon local fish within the Tyne;

• Sea Trout and Salmon are the major fish species within the Tyne, which are susceptible to pH’s below 5.6 (NRA). The acid waters lead to a lowering of the rivers pH and raising the possibility of reducing fish species diversity. Fish are also known to be extremely intolerant of heavy metals, often causing gill related diseases and increasing egg mortality.
• Ochre coating of the river beds and banks blankets benthic environments and reduces natural feeding grounds.

It is widely accepted that the initial flush of water through the mine system will contain the highest concentrations of pollution, after which discharge will progressively reduce in pollutant potency. However, if the concept is to enable frequent flushing of the system during each intense storm, then between storms the acid producing salts will naturally precipitate during a dry period, thus available to dissolve in the next event. This repeat flushing is unlike anything examined previously in the literature, as most mines once the water table has fully rebounded will exhibit a steady/predictable rate of discharge.

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As none of the minor aquifers within Newcastle’s subsurface are used for potable water supplies, minewater pollution of local aquifers is negligible, however, the simple fact that a groundwater source has the possibility to become polluted is something that goes against the Water Framework Directive. Natural acidic neutralisation occurs in sequences which have thick limestone beds within them (such as the Magnesian Limestone below the Durham Coalfield), something the geology beneath Newcastle lacks.

5.4 Remedial Recommendations

Possible minewater outflows have been tentatively identified along both the Ouseburn and the River Tyne near Scotswood. These have the potential to cause localised acidification, unsightly iron precipitates and damage to fresh water ecosystem, thus if true will require remediation. There are two options to treating acidic mine waters; passive and active, both of which will be briefly examined below.

Passive systems are a relatively cheap method of naturalising AMD, as they exploit the landscape, use local materials (agriculture waste and limestone aggregates) while providing a natural looking feature which benefits local wildlife (Younger et al., 1998). The primary drawback of passive systems are the large land requirements which would not suit the urban landscape of Newcastle, coupled with a process whose effectiveness reduces over time. To test the viability of a passive system in the city, current available brownfield site data was obtained from the NCC, and matched with hypothetical outflows of AMD along the river banks. Brownfield sites within 500 m of the river were chosen providing an available area of 0.68 km², of which only 3 sites are of large enough size for a passive system (Fig. 48).

Using minewater chemistry values for analogous workings in the nearby Durham Coalfeld (Younger & Bradley, 1994), flushed waters can be expected to have the following properties: Low pH (4.1-6.5), low alkalinity (0-364 mg/l CaCO₃), high acidity (120 mg/l) relatively low flows (0.001-0.1 m³/s), high iron (1.7-79.8 mg/l) and sulphate (137-1358 mg/l) contents. These properties would allow remediation via either a compost wetland or Reducing and Alkalinity Producing Systems (RAPS) methodologies. Estimates of required areas of each technology are given on the next page:

Compost Wetlands excel at dealing with acidic low flows by utilising anoxic organic compost that stimulates bacterial sulphate reduction, helping raise alkalinity which promotes the right conditions for iron precipitation. (For additional information on the remediation processes see Younger et al., Mine Water: Hydrology, Pollution, Remediation (2002). The design of compost wetlands is derived using the equation:

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A=Qd(Ci-Ct)RA

, where A is the footprint,

Qd

(m²) is the inflow of polluted minewaters (l/s),

Ci

is the minewaters acidity load (mg/l),

Ct

is the target acidity (mg/l) and

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RA

is the area adjusted removal rate for acidity (3.5 g/m²/d). The below areal estimation would require a compost wetland of 0.06 km², well within the available brownfield area available. Younger (2002) suggests this area value to be smaller than required as wetlands often contain islands which improves the sites appeal RAPS also relies on neutralising the excess acid within minewaters, combining the organic matter of compost with a layer of carbonate limestone. This combination increases the rate of neutralisation and hence precipitation of minerals, all while requiring a smaller area (15-40%) to treat minewaters of the same chemical make-up.  RAPS design is based upon a 14-hour residency time within the carbonate layer (Younger et al., 2002). The design calculation is similar to that for a compost wetland, yet taking into account a 14-hour cycle The carbonate layer has a porosity of ~50% and so the volume calculated above is doubled (1512 m³ * 2 = 3024 m³). As the carbonate layer is commonly 0.2 m deep the projected area is much larger, 3024/0.2 = 15,120 m². Finally, as the units design is trapezoidal with a gradient of 3:1 and a total depth of carbonate and compost of 0.7 m, the proposed area for a RAPS is 31,962 m².

Issues with applying passive methods to a pulse of minewater exiting Newcastle’s underground systems are;

• Lacking in available area near to proposed outflows. Only Scotswood Development is suitable, yet is designated a site of a new housing estate.
• All brownfield sites are registered as privately owned and therefore land prices increase, especially within a city.
• Most sites are up slope of the proposed adits, making their siting redundant.
• Large volumes of AMD flush may be of too high a concentration to treat successfully with a passive system, < 800 mg CaCO₃/l (Taylor et al., 2005).
• High outflow rates during a storm event will exceed the carrying capacity of a passive system, < 50 l/s (Taylor et al., 2005).

The above pitfalls suggest that an active system would best be suited to the urban landscape with limited space. While occupying a much smaller footprint, these systems are often expensive to install, with running and maintenance costs spiralling in to the millions (EA, 2005). They do however possess the ability to deal flows over up to 350 l/s while dealing with all types of contaminants at once. A nearby working example of an active treatment plant is sited at the Horden High density sludge plant in County Durham.

6.0  Conclusion

There are undoubtedly enough open voids beneath Newcastle upon Tyne to alleviate surface flood waters, yet, the following exerts from the EU Water Framework Directive emphasize a major issue of the projects concept of injecting flood waters into an open mine environment:

• (Article 4b) “Member States shall implement the measures necessary to prevent or limit the input of pollutants into groundwater and to prevent the deterioration of the status of all bodies of groundwater”

As the project requires surface water input into a dry mine void, this will certainly lead to dissolution of acid producing salts and therefore increasing pollutant concentrations within the groundwater.

• (26) “in addition to the requirements of good status, any significant and sustained upward trend in the concentration of any pollutant should be identified and reversed”.

Both groundwater and surface water quality has steadily increased across the UK since the introduction of the WFD, and consequently a stimulus of acidic mine water conflicts this ethos.

• (51 d) “ensures the progressive reduction of pollution of groundwater and prevents its further pollution, and”………. “thereby contribute to a significant reduction in pollution of groundwater”.

Again, a system of periodic flushing of surface water into open voids will undoubtedly spearhead fluxes of highly concentrated acid mine waters and thus directly against the WDF.

Conceptually the project draws together available information from the literature, but lacks the realisation of a numerical model for a comparative analysis. It is hoped that the pitfalls, as well as data requests to aid in future study of what is a complex and poorly understood topic. It should be expected that mineworkings will have a heavy influence upon the flow regime, yet unfortunately a lack of digitised network linear data limits the proposed methodology set out at the beginning of this project.

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As highlighted through the report, MODFLOW-USG is well suited to the project, and as literature grows, adaptation of karst network methodologies should allow for a working model to be developed. However, as academic resources within the university have a strong knowledge of the SHETRAN model, it may be advised for future projects to utilise this resource until knowledge of MODFLOW-USG becomes more widely available/accessible. As the CA suggests that regional groundwater flow to flow south eastward, it is proposed that a pulse of flood waters into a mine system would move along strata towards the rivers.

6.1  Future Work

As the initial scope of the project soon became too great for a 4-month Master’s dissertation, the project lays the groundwork for several possible studies while highlighting some of the concepts major pitfalls. A particular point of interest which would come out of a numerical model is confirmation of groundwater flow through Newcastle’s mineworkings. Generally, this is accepted as flowing with the strata to the SE and interacting with the River Tyne, yet physical field studies including tracers and seepage investigation along the banks of the Tyne may fortify this hypothesis.

Confirmation of water table stabilisation was another objective that unfortunately was missed due to unavailability of additional borehole data from the EA. This data was requested in May, however, no response was provided. Although, levels up to Feb-2016 were already available and appeared to show a relatively stable water table (Crawley, 2016), this should be confirmed in future studies.

6.1.1 Refinement of 3D Mineworkings (CA)

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Potential starting points for a refined MODFLOW-USG model would require both borehole and mineworking tunnel data, something that the Coal Authority is yet to digitise. Borehole data is invaluable to the construction of a MODFLOW model as they are the primary data set utilised in most tutorials for layering complex stratigraphy. As highlighted at the beginning of the project, the CLN add-on for MODFLOW requires a connected network of points to transmit non-Darcian flow which although laborious could be completed using the Newcastle Mining Museums extensive paper maps and literature.

Additional techniques for groundwater investigation include;

• Geophysical scanning of the underground workings, however, this tends to require exuberant funding.
• Dumpleton et al., (2001) successfully used the VULCAN 3D modelling package to locate and define discrete mine connections and potential surface outflows.
• Tracers, commonly used for pollutant plumes could be added as an inexpensive way of observing outflows along the many seepage faces on the Tyne.

Tracer technology could be used to aid in tracking groundwater flow as well as potential seepage points along the river banks. First instances of tracer testing within mines were to document connections between groundwater and the mine (Wolkersdofer, 2006). This resource has since expanded to determine hydraulic connections, flow direction, groundwater velocities and residence time. Tracer type is limited by the groundwaters velocity (residence time) and chemical composition, therefore using an analogous groundwater composition from the neighbouring Durham Coalfield (Younger & Bradley, 1994), a florescence tracer (Aldous & Smart, 1988) should be suitable for the expected anoxic, low pH coal mine waters. Florescent dyes are relatively cheap compared to other tracer options (e.g. radioactive), are non-poisonous, able to ‘sorb’ onto mine particles making outflow easily detected. Further information on choosing the correct tracer can be found in Wolkersdofer (2006).

6.1.2 MODFLOW Model Possibilities

To take the project further and develop a MODFLOW model will require a student with a background in MODFLOW. It is proposed that a smaller study area be taken on using a simplified linear mine network. If the student knows that digitised network data is not available then plans can be made to create this at the early stages of the project.

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The MODFLOW code contains many extensions, one of which is pollution transportation and contaminant flow model. This was merely glanced over during reading of the MODFLOW manual and requires investigation.

Once a working model is achieved, it has the potential to lend itself to transient modelling to examine changing head due to seasonality. The prospect of climate change scenarios in line with the proposals of the IPCC. As well as the integration of an urban server network into the model.

6.1.3 Current Usages

The initial aim of this project was to investigate the viability injecting water into an anthropogenically altered groundwater system which would appear possible if not for the Water Framework Directive. The project does lay the groundwork for a numerical model creation, highlighting the do’s and don’ts of its development.

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As advised by Bruce Misstear during the dissertation presentations (July 2017), the project proved beyond the scope of a Master’s thesis and may suit post-doctoral research or divided between multiple Masters students. Further, discussions and involvement from the EA and CA would also be welcomed.

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