IAH NSW, Groundwater in the Sydney Basin Symposium, Sydney, NSW, Australia, 4-5 Aug. 2009, W.A.Milne-Home (Ed) ISBN

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1 Keynote Paper Hydrological Changes due to Longwall Mining in the Southern Coalfield, New South Wales, Australia J.Jankowski a* a Sydney Catchment Authority, Penrith, NSW, 2751, Australia, Tel: (2) , Fax (2) , jerzy.jankowski@sca.nsw.gov.au *Corresponding author Abstract Longwall mining may impact surface water and groundwater as a consequence of mininginduced subsidence, which includes the vertical and horizontal deformations and displacement of the ground mass. As a result of fracturing riverbeds and rockbars, surface flow is diverted to subsurface routes and surface water-groundwater connectivity is enhanced. The interaction between surface water and groundwater can occur laterally and longitudinally to a stream. Several types of interactions are possible based on the extent of fracturing in the aquifer; from a single recharge-discharge zone to multiple recharge-discharge zones. A reduction in streamflow may not only be the result of fracturing streambeds and rockbars in the main stream overlying an active longwall mine; mining-induced fracturing can extend across the catchment and its tributaries, generally bounded by the limit of subsidence. Increased fracturing allows rainfall to infiltrate and recharge fractured aquifers, reducing runoff available for recharging streams. Although rainfall recharge to the shallow aquifers can increase, groundwater levels can also decline due to the mining-induced fracturing of the rock mass, causing the dewatering of shallow aquifers and reducing baseflow discharge. Fracturing of the banks of streams and tributaries can also reduce streamflow during high flow conditions. Streamflow reduction is also an effect of the spatial distribution and density of fracture networks, changes in porosity and permeability of the subsurface rock mass, changes in groundwater storage capacity, modification to baseflow discharge and alteration of the hydraulic gradient near the streams. Keywords: Longwall mining, surface water-groundwater interaction, streamflow loss, fractured aquifer Introduction Streamflow reduction, including streamflow loss, is a net flow reduction along a stream that can be associated with natural or anthropogenic hydrological conditions. In catchments with longwall mining activities, the reduction in streamflow can be the result of complex hydrological processes that affect the catchment water balance due to mininginduced fracturing and subsidence. This subsidence can result in the fracturing of streambeds and rockbars, alteration of the connectivity between surface water and groundwater, reduction in baseflow discharge to streams and reversal of the hydraulic gradient. Concern over the potential impact on water supplies and the maintenance of stream function has increased following the observed surface water-groundwater connectivity in areas where mining-induced subsidence has led to declines in baseflow discharge to

2 streams. There have been various studies undertaken above active longwall mines, providing some insight into mining-induced subsidence on the temporary or permanent loss of streamflow and the increase in surface water-groundwater interaction. Streamflow depletion associated with longwall mining was observed and detailed analysis provided for the Appalachian Coalfield, USA (Carver & Rauch, 1994; Dixon & Rauch, 1988, 199; Tieman et al., 1987, 1992); Utah Coalfield, USA (Slaughter et al., 1995) and East Midlands, England (Shepley et al., 28). A reduction in streamflow has also been identified in a Southern Coalfield catchment of NSW, Australia, where mining induced-subsidence has caused the enlargement of existing fractures, the development of new fractures (including the fracturing of streambeds and rockbars), the separation of bedding planes and changes in channel geometry. These changes have lead to a diversion of surface water to the subsurface, dry streambeds and disconnection between surface water and groundwater (Jankowski, 27; Jankowski & Spies, 27; Jankowski et al., 28; Kay et al., 26). Conceptual surface water-groundwater interaction According to Larkin & Sharp (1992), the stream-aquifer system can be classified based on the predominant regional groundwater flow component: (1) underflowcomponent dominated, with groundwater flow longitudinally to a stream and in the same direction as stream flow; (2) baseflow-component dominated, with groundwater flow moving laterally to or from a stream depending on whether the stream is gaining or losing; or (3) a combination of both. These flow paths into the stream create 3D physicochemical patterns controlled by the groundwater flow pattern (Brunke & Gonser, 1997). The stream-aquifer classification was determined for alluvial aquifers, however it is generally transferable to shallow fractured rock aquifers impacted by mining, where intensive fracturing produces a similar flow pattern. The above three groundwater flow types are postulated in a Southern Coalfield catchment impacted by longwall mining, through the development of new fractures, enlargement of existing fractures, separation of bedding planes and the modification of stream topography (Jankowski, 27). Streamflow may be permanent or temporary based on the following scenarios: Permanent flow occurs when the: stream is connected-gaining and there are baseflow contributions from regional aquifers; size and distribution of the surface fracture network is small, limiting surface water infiltration; capacity of the subsurface system to store water is lower than the streamflow infiltration rate. Temporary flow occurs when the: baseflow contribution is small and unreliable; size and distribution of the surface fracture network is large, allowing increased surface water infiltration; capacity of the subsurface system to store water is higher than the streamflow infiltration rate. In the Southern Coalfield, for example, observed maximum subsidence may be up to 2.2 m (Dendrobium Colliery Area 2) (BHP Billiton, 28) and observed maximum upsidence may be up to 197 mm (Waratah Rivulet, Metropolitan Colliery) (HCPL, 26).

3 Mining induced subsidence causes topographic and structural modifications to streambeds and the drainage basin, generally bounded by the angle of draw (subsidence to 2 mm). Fracturing of streambeds (Fig. 1) and rockbars (Fig. 2) causes surface water to divert to subsurface routes and interact with groundwater. In the Southern Coalfield, surface water typically flows vertically through fractures and horizontally through bedding planes. Recharge to the shallow sandstone aquifer also occurs through joints, veins and large cavities, with baseflow discharge occurring through fractures (flow is often under artesian pressure) and bedding planes. Depending on the connectivity between the fracture network and bedding planes, a complex groundwater flow system can develop in the sandstone aquifer. In natural incised sandstone valleys, groundwater flows towards a stream and surface watergroundwater interaction represents a connected-gaining system. Mining impacts can cause streamflow losses along sections of streams, with surface flow diverted into the subsurface and the potential for a net reduction in flow at the end of the hydrological flow system. This enhanced surface water-groundwater connectivity along streams can result in the system changing from connected-gaining, to connected-losing and to disconnectedlosing. Lateral and longitudinal flow around and along a stream and the location of recharge and discharge zones are modified with subsidence impacts on streambeds. Figure 1 Diversion of surface flow into the subsurface due to fracturing of streambeds natural versus impacted systems

4 Figure 2 Diversion of surface flow into the subsurface due to fracturing of rockbars natural versus impacted systems Several conceptual scenarios have been identified by Jankowski (27) to describe surface water-groundwater connectivity, subsurface flow pattern, recharge/discharge zones and mixing between surface water and groundwater. These scenarios, based on lateral and longitudinal surface water-groundwater flow, are discussed below. Lateral surface water-groundwater flow in a longwall mining impacted catchment depends on the hydraulic gradient. 3D groundwater flow patterns depend on the number of aquifers and aquitards, distribution and connectivity of fractures and bedding planes, and can include the following scenarios: Before mining groundwater flow is typically towards the stream with a steep hydraulic gradient and baseflow discharge connected-gaining stream. During mining there may be impacts to the wider catchment, with groundwater flow towards the stream yet the hydraulic gradient is flatter and baseflow discharge is reduced connected-gaining stream but with potential for reduced gains. During mining there is a flat water table and/or slightly changing hydraulic gradient and a possibility of groundwater flow through a stream. During mining there may be a reversal of the hydraulic gradient, where surface water recharges the subsurface and groundwater flows away from the stream connected-losing stream. Part of a stream is above an extracted longwall panel and part of the stream in an unmined area, potentially causing a reversal of the hydraulic gradient in the mined area and the hydraulic gradient to slope towards the stream in the unmined area connected-gaining and connected-losing or flow through a stream. During mining there may be significant mining impact with fracturing of streambeds, fracturing of the strata over a substantial thickness and impact to the wider catchment, resulting in the groundwater table lowering below the streambed, a lack of surface flow or reduced surface flow during dry

5 periods, and reduced surface flow during rainfall due to increased groundwater recharge disconnected-losing stream-aquifer system. Longitudinal surface water-groundwater flow can have several recharge-discharge zones present along the streambed causing surface flow to recharge the subsurface and potentially discharge downstream. The flow systems which can result can include the following scenarios: A simple shallow flow system along a stream with multiple separate subsurface flow paths, developed along preferential bedding plane(s) and fracture(s). A simple deeper flow system along a stream with multiple separate subsurface flow paths developed along preferential bedding plane(s) and fracture(s). Complex surface water-groundwater interaction with shallow and deeper subsurface flow paths, developed along preferential bedding planes and fractures with many locations of surface water and groundwater mixing zones. Complex surface water-groundwater interaction with multiple rechargedischarge and mixing zones, where flow occurs along a single preferential bedding plane and number of fractures. Complex surface water-groundwater interactions with multiple recharge and mixing zones and a single discharge zone, where the flow travels along a single preferential bedding plane and a number of fractures. Complex surface water-groundwater interaction with a single recharge zone and multiple discharge zones, where the flow travels along a single preferential bedding plane and number of fractures. Complex 3D surface water-groundwater interaction with multiple recharge and mixing zones, and flow travelling along several bedding planes and fractures. No discharge zone is locally present; groundwater may leave the catchment as regional flow or flow to a lower aquifer system. Streamflow reduction Evidence from surface flow A lack of detailed baseline hydrological monitoring data is the main obstacle to adequately assessing the impact of mining on catchment hydrology, however a range of methods have been used to assess streamflow. Figure 3 shows the streamflow data from the main stream in a Southern Coalfield catchment impacted by longwall mining. Although there is no pre-mining data, one method used for assessing the streamflow data is based on subtracting the upstream streamflow from downstream streamflow, which has been used by others, such as Tieman et al. (1987).

6 1 Upstream gauging station (G1) Gauging station in the mining area (G2) Downstream gauging station (G3) Streamflow Volume (ML/day) /4/27 3/6/27 25/9/27 21/12/27 17/3/28 12/6/28 7/9/28 3/12/28 28/2/29 Figure 3 Comparison of streamflow upstream (G1), in the mining area (G2) and downstream of the mining area (G3)

7 Flow Difference ML/day (G2-G1) Gain Loss -3 4/4/27 3/6/27 25/9/27 21/12/27 17/3/28 12/6/28 7/9/28 3/12/28 28/2/29 Data 2 Flow Difference ML/day (G3-G2) Loss Gain -15 4/4/27 3/6/27 25/9/27 21/12/27 17/3/28 12/6/28 7/9/28 3/12/28 28/2/29 Figure 4 Flow difference between mining area (G2) and upstream (G1) gauging stations (left), flow difference between downstream (G3) and mining area (G2) gauging stations (right) As shown in Figure 3, the upstream gauging station (G1), which is located on the upstream edge of the mining affected area and is likely to represent close to natural flow conditions, has lower flow during dry periods compared to the other gauging stations (G2 is located on the downstream edge of the mining area and G3 is located downstream of the mining area). This lower flow is expected, as the catchment area increases downstream and there is likely to be increased volume contribution to G2 and G3 from additional runoff, flow from tributary creeks and baseflow discharge. During periods of prolonged dry weather, the reduction in surface flow becomes visually evident as streamflow is diverted into the subsurface and there are sections of the stream which are dry. When the flow data from G1 is subtracted from the flow data from G2, it appears that typically the low flows at G2 are higher than the low flows at G1 (Fig. 4). Increasing flow downstream is due to incremental contributions from the catchment and baseflow discharge to some or the entire stream length between these two gauging stations. When

8 the flow data from G2 is subtracted from the flow data from G3, the volume of water at the downstream location is lower than the volume of water at the upstream location (Fig. 4). The sharp losses shown in Figure 4 typically occur: just before large rainfall events and may represent a lag in travel time; and during high flow conditions and may represent errors with the data (rating curves appear to be less reliable during these conditions). Although no data was collected prior to the commencement of mining, the monitoring data available indicates there is a loss of surface flow. Evidence from shallow groundwater Groundwater level data obtained from shallow piezometers along the stream is shown in Figure 5. The groundwater levels at six locations representing the upstream (mined in 23-24) (GW5 and GW6), mining area (GW3 and GW4) and downstream (edge of mining above the rib zone) (GW1 and GW2) conditions were compared with the streambed elevation and indicate the connectivity between surface water and shallow groundwater (Jankowski et al., 28). GW5 and GW6 shows the disconnected-losing part of the stream, where the groundwater level is below the lowest streambed elevation. This area was mined in the past and currently represents a nearly recovered part of the hydrological system. Fractures, joints and bedding planes still provide pathways for subsurface flow, however the openings are smaller than they would be above the active mining panel. There is usually surface flow in the stream at this location, however surface flow also recharges the subsurface system with a relatively fast infiltration rate, indicating a high hydraulic conductivity of the aquifer. It is expected that lateral flow dominates natural rainfall recharge of the aquifer, whereas longitudinal flow is expected to dominate subsurface flow. At GW3 and GW4 the stream is usually connected-gaining, with the groundwater level above the lowest stream bed elevation. It is expected that this part of the stream is in the compressional phase of subsidence, where fractures have partially re-closed, there is limited vertical and horizontal extension of fractures and bedding planes, the groundwater level has partially recovered and groundwater discharges through vertical fractures under pressure. However, this area appears to be undergoing some fracturing and incremental impact by subsequent longwall panels. During low streamflow conditions in January-February 29, the stream become connected-losing as the groundwater level went below the streambed elevation. This may be associated with the relatively low rainfall conditions that prevailed in the end of 28 and early 29 as well as an incremental effect of longwall panels LW15 and LW16 on the catchment surface water and groundwater. At GW1 and GW2, the groundwater level remained below the lowest streambed elevation, indicating a disconnected-losing part of the stream. Longwall mining has not occurred at this location, however the piezometers are located over the headings at the edge of an extracted longwall panel and is within the rib area covered by a 35 o angle of draw. This part of the stream is disconnected-losing, as subsidence is likely to have developed significant openings with a high capacity to allow the inflow of surface water.

9 GW1 Disconnected-losing system Streambed elevation GW2 Disconnected-losing system Streambed elevation /2/7 17/5/7 15/8/7 13/11/7 11/2/8 11/5/8 9/8/8 7/11/8 5/2/ /2/7 17/5/7 15/8/7 13/11/7 11/2/8 11/5/8 9/8/8 7/11/8 5/2/9 216 GW3 Connected-gaining system GW4 Connected-gaining system Bypass pumping during stream remediation Streambed elevation Bypass pumping during stream remediation Streambed elevation /2/7 17/5/7 15/8/7 13/11/7 11/2/8 11/5/8 9/8/8 7/11/8 5/2/9 16/2/7 17/5/7 15/8/7 13/11/7 11/2/8 11/5/8 9/8/8 7/11/8 5/2/ GW5 Disconnected-losing system Streambed elevation GW6 Disconnected-losing system Streambed elevation /4/7 3/7/7 1/1/7 3/12/7 29/3/8 27/6/8 25/9/8 24/12/8 4/4/7 3/7/7 1/1/7 3/12/7 29/3/8 27/6/8 25/9/8 24/12/8 Figure 5 Groundwater levels upstream (GW5 and GW6), in mining area (GW3 and GW4) and downstream (GW1 and GW2) (modified from Jankowski et al. 28) Conclusions The method used of subtracting the upstream streamflow from downstream streamflow is appropriate for providing an initial indication on whether or not there is

10 streamflow loss. However, in mining impacted catchments in particular, the method is a simplification, due to the complex hydrological processes that occur. The complexity of the hydrology results in difficulties in assessing stream flow losses, as there can be decreases in surface runoff, reduction in baseflow discharge to tributary creeks and streams, and leakage from the shallow aquifer to deeper regional aquifers. This system, dominated by surface water-groundwater interaction, is very difficult to monitor. Vertical and horizontal ground movements result in the development of new fractures and the enlargement of existing fractures and bedding planes. Pathways are created, allowing surface water to recharge the shallow subsurface, causing a reduction in streamflow and decreased baseflow discharge to the main stream. Losses to deeper regional aquifer have not been monitored or accessed. Reduction in streamflow depends on surface fracturing and the hydraulic connectivity between surface flow, shallow aquifers and deeper regional aquifers. Shallow piezometers located near the stream indicate that shallow groundwater close to the stream is affected by subsidence, causing the majority of groundwater levels to be below the streambed, causing the stream to be disconnectedlosing with the diversion of surface water into subsurface voids. If the shallow aquifer becomes connected to the deeper regional aquifer, it is possible that this water will be lost from the immediate catchment. Acknowledgements The author thanks Andrea Madden, Penny Knights and John Ross of the Sydney Catchment Authority for comments on an earlier draft of the paper. References BHP Billiton 28, Dendrobium Colliery Area 2, Longwall 3, End of Panel Report for Longwall 3 at Dendrobium Colliery, Comur Consulting Pty Ltd, March 28. Brunke, M. & Gonser, T. 1997, The ecological significance of exchange processes between rivers and groundwater, Fresh-water Biology, vol. 37, pp Carver, L. & Rauch, H. 1994, Hydrogeologic effects of subsidence at a longwall mine in the Pittsburgh coal seam, Peng, S.S. (ed) Proceedings of the 13th Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, USA, August 2-4, 1994, pp Dixon, D.Y. & Rauch, H.M. 1988, Study of quantitative impacts to ground water associated with longwall coal mining at three mines sites in the northern West Virginia area, Peng, S.S. (ed) Proceedings of the 7th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, USA, 3-5 August 1988, pp Dixon, D.Y. & Rauch, H.W. 199, The impact of three longwall coal mines on streamflow in the Appalachian Coalfield, Proceedings of the 9th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, USA, 199, pp HCPL 26, Metropolitan Colliery, Longwall Subsidence, End of Block Report, Longwall 12, Helensburgh Coal Pty Ltd.

11 Jankowski, J. 27, Surface water-groundwater interactions in a catchment impacted by longwall mining, Li, G. & Kay, D. (eds) Proceedings of the 7th Triennial Conference on Mine Subsidence, Wollongong, November 26 27, 27, pp Jankowski, J., Madden, A. & McLean, W. 28, Surface water - groundwater connectivity in a longwall mining impacted catchment in the Southern Coalfield, NSW, Australia, Lambert, M., Daniell, T. & Leonard, M. (eds) Proceedings of the Water Down Under 28, Adelaide, SA, April 14 17, 28, CD-ROM, 12 pp. Jankowski, J. & Spies, B. 27, Impact of longwall mining on surface water groundwater interaction and changes in chemical composition of creek water, Ribeiro, L., Chambel, A. & Condesso de Melo, M.T. (eds) Proceedings of the XXXV IAH Congress, Lisbon, Portugal, September 17 21, 27, CD-ROM, 1 pp. Kay, D., Barbato, J., Brassington, G. & de Somer, B. 26, Impacts of longwall mining to rivers, and cliffs in the Southern coalfields, Proceedings of the 7th Underground Coal Operators Conference, Wollongong, July 5-7, 26, pp Larkin, R.G. & Sharp, J.M. Jr. 1992, On the relationship between river-basin geomorphology, aquifer hydraulics, and ground-water flow direction in alluvial aquifers, Geol. Soc. Am. Bull., vol. 14, Shepley, M.G., Pearson, A.D., Smith, G.D. & Banton, C.J. 28, The impacts of coal mining subsidence on groundwater resources management of the East Midlands Permo-Triassic Sandstone aquifer, England, Quart. J. Eng. Geol. Hydrogeol., vol. 41, Sidle, R.C., Kamil, I., Sharma, A. & Yamashita, S. 2, Stream response to subsidence from underground coal mining in central Utah, Environ. Geol., vol. 39, Slaughter, C.B., Freethey, G.W.& Spangler, L.E. 1995, Hydrology of the North Fork of the Right Fork of Miller Creek, Carbon County, Utah, before, during, and after underground coal mining, Water Res. Invest. Rep., 95-25, USGS, 56 p. Tieman, G.E., Rauch, H.W. & Carver, L.S. 1987, Study of dewatering effects at a longwall mine in Northern West Virginia, Proceedings of the 3rd Subsidence Workshop due to Underground Mining, West Virginia University, Morgantown, WV, USA, 1987, pp Tieman, G.E., Rauch, H.W. & Carver, L.S. 1992, Study of dewatering effects at a longwall mine in northern West Virginia, Proceedings of the 3rd Workshop on Surface Subsidence Due to Underground Mining, COMER, West Virginia University, Morgantown, WV, USA, pp