Samrit Luoma1 & Kaisa Turunen2, 1Geological Survey of Finland, P.O. 96, FI-02151 Espoo, FINLAND, samrit.luoma(at)gtk.fi; 2Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio FINLAND, kaisa.turunen(at)gtk.fi
Mining operations often impact both water quality and availability. Moreover, because watersheds do not follow the geographic boundaries of a mine site, the effects on water systems extend beyond the mine. Recent accidents related to water management at mine sites have raised concern regarding the ability to adjust mining processes based on variable hydrological conditions and adapt water management tools and models for accurate water balance calculations. Moreover, best practice recommendations are needed for designing sufficient water management infrastructures to handle variable water volumes and to separate mine waters from waste facilities and fresh waters. To minimize environmental impacts of mining, all phases of mine operation planning require accurate knowledge of the variables associated with hydrological and hydrogeological conditions at a mine site. This includes consideration of the mining operation specifics, hydrological control structure design and best management practices for site wastes.
The aim of this Closedure hydrological and hydrogeological research page is to outline the methods commonly used to characterize natural system waters at a mine site, including the methods for characterizing surface water hydrology, ground water hydrogeology, and surface water-groundwater interactions. The characterization, handling, and treatment of mine waters are discussed in the Closedure mine water treatment page.
Topography, geology and geomorphology
Since the infiltration, surface runoff and groundwater recharge are all controlled by the characteristics of the soil and bedrock, comprehensive information about the surface and near surface geology and morphology of the site is crucial for quantifying and evaluating the hydrological and hydrogeological characters of the site. At closure phase, this kind of data is already available due to the characterization of the ore and environmental studies, but especially the topsoil characteristics have been changed during the mine operations and further evaluation may be needed. Good quality topography, soil and bedrock maps are the basis of any water management study and give often enough information on the hydrogeological characteristics of the site. Often the mine has conducted also geophysical measurements that reveal for instance the fracturing of the bedrock. Although one has all this data, a deep understanding of how these components affect the water courses developed and continue to change in long-term is essential.
Mining operations involve two main hydrologic components: natural and mine water systems. Natural waters are associated with the natural hydrological cycle, such as groundwater and meteoric water from precipitation, snowmelt, evaporation, and runoff. During the mine-life-cycle, sufficient and accurate knowledge and characterization of baseline hydrogeological parameters in the mine site are crucial for establishing baseline hydrological conditions for the prediction of drainage release and transport, and monitoring of the environmental conditions and potential impacts. The hydrological parameters are needed also for impact evaluations and the proper design of detention structures, diversions, culverts, pregnant ponds and barren ponds, tailings dams, and other facilities controlling waters at mine site (Morton and Mekerk 1993, EPA 2003). Moreover, the climatic and hydrogeological characteristics of the site control the behaviour of constituents present in mine drainage which is transported through the environment to the receiving water systems (e.g. Wolkersdorfer 2008, Salonen et al. 2014).
Precipitation and evaporation
Precipitation and temperature are the main components of the climate data that directly control the amount of water in the watershed and the mine site. Precipitation, such as rainfall and snowmelt, has an important role on surface flow, spring flow, seepage and groundwater recharge and discharge. The climatic and hydrogeological characteristics of the site control the behaviour of constituents present in mine drainage which is transported through the environment to the receiving water systems (e.g. Wolkersdorfer 2008, Salonen et al. 2014). In mine water management study one should have at least the following data related to precipitation:
- The amount of precipitation, mm
- The intensity of the precipitation, mm/h
- The duration of the precipitation, minutes, hours or days of rain
- The annual average of 5 and 10-year dry or wet period data
- The return period of an extreme precipitation event that has a return period interval of T years
Recently occurred accidents related to water management at mine sites have raised a question on the mines ability to adjust process parameters to variable hydrological condition especially during extreme events and return periods. The return period is expressed in years of specific events and it is used for example for the accurate sizing of a major diversion ditches or tailings ponds. The return period means an event when, for example, a massive heavy rain will occur on average every fifty years at specific location and the water structures at the mine site should be able to handle the exceeding water volumes. The return period (Tr) can be calculated with the following formula, where p is the probability, of the event being equalled or exceeded in any year:
Thus for a precipitation event with a 50-year recurrence interval, the probability is two percent that that event will be equalled or exceeded in a given year. The water management structures should be designed for their lifetime and thus, also the return periods differ between structures. For example, the dewatering structures are often used only during mining operations, whereas the spillways of tailings facilities are planned to be in operation even for decades after mine closure. Thus, 100-year extreme events are not needed to be included in the design of both structures (Ritter 1992, He and Wilkerson 2011).
The amount of precipitation varies temporally and spatially and can be measured at the mine site using rain gauges. It is important to have site specific information for the watershed area of the site and at least one meteorological station should be installed at the mine site for monitoring the climate data e.g. temperature, precipitation (rainfall and snowmelt), humidity, wind speed and direction, snow survey, pan evaporation and radiation (Golder Associates 2011). Information of the regional precipitation and climate in Finland is available in the webpage of the Finnish Meteorological Institute (www.fmi.fi). The real-time measurements for hydro-meteorological data in the watershed area and the corresponding forecasting system – the Watershed Simulation and Forecasting System (WSFS) (Vehviläinen 1994) can be found in the webpage of the Finnish Environment Institute (https://www.environment.fi/en-US/Waters/Hydrological_situation_and_forecasts).
Finland is located in the northern hemisphere, and is a part of the Eurasian continent’s coastal zone, with characteristics of both a maritime and continental climates. Winters in Finland are cold and wet, but the weather can change rapidly. Rainfall is moderate around the year (Finnish Meteorological Institute 2014). Table 1 presents average amounts of precipitation, runoff, and evaporation in Finland, which are the basic components of the water balance (Karhu 2005). The water balances in Finnish mines are usually net positive (Haanpää 2013), typically with 40-60% positive overbalance (Salonen et al. 2014). Evaporation is the process in which water in its liquid or solid state is converted to water vapour. This process is controlled by 1) the available energy at the evaporating surface, and 2) the humidity of the air above the evaporating surface (Kokkonen et al. 2011). The volume of water loss via evaporation is calculated based on the monitoring data and the wetted surface area in the mine site.
Table 1. The average amounts of precipitation, runoff, and evaporation in Finland (Karhu 2005).
|August is the most rainy season, March the most dry
|Overall runoff, Q
|A share of spring runoff is around 30-50 %
|In Northern Finland the evaporation rate is only around 25-50 % of the average
A snow melt is an essential part of quantifying the hydrological cycle in cold regions. A significant proportion of the annual runoff and groundwater recharge may occur during a short period in spring promptly after snowmelt. Typical purpose for snow modelling is to provide an estimate of snowmelt input together with the amount of precipitation (from rainfall) for stream flow forecasts, which are necessary for flood warning systems and water regulation decision issues (Kokkonen et al. 2011). The prediction of runoff from snow melt is complicated by other hydrological factors such as groundwater storage, soil-moisture deficiency, and the amount of precipitation that occurs during runoff periods (Kokkonen et al. 2011, EPA 2003).
Surface run-off and infiltration
The amount of runoff that occurs is a function of the intensity and duration of the precipitation. In addition, the runoff volumes depend on soil moisture and its infiltration capacities. For instance, a short but intensive rain period creates more runoff than a long but light-rain period, since the soil is not able to infiltrate the extensive water volumes in short period. Moreover, the dryer the soil the more infiltration and less runoff will occur. Thus, after an intense drought the infiltration is exceeding the runoff (EPA 2003). The peak flow of surface runoff during spring can cause overflow and flooding, while low water levels in summer can pose a risk of water shortages. Peak runoff rates from a basin or watershed can be estimated by the following formula:
Q = C i A (A-1)
where Q is the peak runoff rate, C is a dimensionless coefficient, i is the rainfall intensity, and A is the drainage area of the basin (EPA 2003).
The construction of the mine site facilities such as roads, buildings and tailings dams, changes the surface conditions and thus also the hydrological characteristics of the site. For instance, road constructions usually increase the compaction of the soil resulting in reduced infiltration, but increased surface runoff. This in turn, may lead to an increase of a peak flow and the total stream discharge associated with a given storm event. The increased water volumes in a stream may erode the banks and channels, resulting in increased sediment loads in receiving water systems. According to Ritter (1992) the infiltration capacity of newly reclaimed areas is nearly an order of magnitude lower than undisturbed forest soils. However, the infiltration capacity tends to recover after 3 to 4 years after reclamation of the mine site (Jorgensen and Gardner 1987).
Infiltration is the primary regulator of hydrology in drainage basins and distributing rainfall between surface and subsurface hydrologic systems (Jorgensen and Gardner 1987). Together with evaporation, infiltration affects also the rate and quantity of surface water runoff. Moreover, the groundwater quality is at risk if the infiltration water percolates through waste facilities. Infiltration depends on the soil hydraulic conductivity, which in turn depends on soil type and structure. In addition, the slope geometry including inclination and surface roughness regulates the infiltration and surface runoff conditions. Vegetation prevents overland flow and affects contents of pre-precipitation soil moisture. The infiltrometer tests provide a potentially useful and cost-effective means of evaluating hydrological characters at mine site (Ritten 1992, EPA2003).
The location in northern hemisphere and the characteristics from both a maritime and continental climates affect seasonal fluctuation of water in watershed, seasonal infiltration and groundwater recharge. Groundwater level contains spatial and temporal variations depending on several factors including hydrogeological condition of aquifer, surface water and groundwater interactions, groundwater extraction, and groundwater recharge which is directly regulated by climate conditions. In Finland, due to the cold and snowy winters, groundwater recharge occurs mainly twice a year, during spring (late March-early April) and late autumn (November-early December), following infiltration of snow melt and rainfall. Increased groundwater recharge in spring increases also the groundwater table level closer to the ground surface unless groundwater is discharged to low-lying areas. A shorter infiltration depth or an existing wetland area will pose a contamination risk for groundwater quality. Thus, groundwater monitoring program is needed for regular and long term monitoring of groundwater levels, surface water levels throughout the year and it should be measured together with the local climate data. The information of groundwater and surface water interaction in different seasons is important for the water management and planning. This data can be obtained from the field investigation and the long term monitoring which is needed to fully characterize the hydrological behaviour of the aquifer, and the analysis of contaminant movement in the mine site. However, under the changing climate conditions, the long-term effects of climate change on the annual temperature range, total precipitation, seasonal variation, peak precipitation events, evaporation, and hydraulic routing are difficult to predict (Brixel et al. 2012). Thus, it is necessary to select design parameters for water management structures, which are based on conservative interpretation of historic records and with consideration for the changes that may occur in the future (NWT 2007).
Groundwater quality analysis and monitoring is also important to follow up the geochemical processes including attenuation to establish long term groundwater protection. Moreover, the long-term performance of the groundwater interception systems and interaction with surface water should be assessed and further evaluated if required using the groundwater model developed during the operational stage of the mine waste disposal facility. The prevention of groundwater from contamination from mine site, e.g. seepage or leachate of mine drainage water from waste rock piles or unlined tailing dams should be designed from the beginning of mining operation. This includes installation of groundwater monitoring network prior to and during the operation of a mine waste disposal facility in order to generate historical records of the parameters of interests (Brixel et al. 2012). The bottom of the mine waste facilities are usually covered with liner and the wastes itself covered with different kinds of structures to prevent the infiltration through the wastes and to reduce seepage from the tailings to the groundwater. However, a mine waste disposal facility should not rely only on a liner, because in the long term liners will decay and fail to control seepage from the tailings to the groundwater system (Brixel et al. 2012). The long term transient seepage analyses can be predicted by using analytical seepage modelling. The groundwater monitoring and quality analysis is described in Closedure monitoring page and the cover design and waste management in Closedure Wastes and waste facilities section.
The management of large quantities of water has been problematic and these problems mainly emerge because mine sites’ water balances have not been adequately assessed in the planning stage of mines. In addition, recently occurred accidents related to water management at mine sites have raised a question on the mines ability to adjust process parameters to variable hydrological condition and adaptation of suitable water management tools and models especially related to water balance calculations. A water budget is simply a statement of mass balance for hydrology in watershed area. The water balance is used for calculating the flow and storage changes in natural water system such as rivers, lakes, drainage basins, and aquifer by following equation (USDA 2007):
Inflow = Outflow ± Change in Storage,
where water inflow includes precipitation, surface water flow into basin, imported water, groundwater inflow and infiltration; water outflow consists of evaporation, evapotranspiration, surface water outflow, exported water, groundwater outflow; and the change in water storage is either increased or decreased in e.g. snowpack, unsaturated soil zone, streams, rivers, reservoirs, or aquifer. At a mine site, the water fractions from flow into and out of the site commonly consist of:
- surface water
- mine pit water
- precipitation runoff
Water balance acts as a starting point for all quantitative water analyses. Moreover, it is an important tool in assessing the current availability of water in area and predicting the future trends. It can also be used to consolidate decision making in water management (Moriarty et al. 2007). The water balance in the mine site can be calculated in a regional scale or a sub-basin scale. For example, a generalized water balance equation for a pit lake can be summarized as follows (Gammons et al. 2009):
[P + SWin + GWin] = [E + T + SWout + GWout] ± ΔS
where P is direct precipitation falling on the surface of lake; SWin is the sum of surface water inputs e.g. diverted streams, storm flow, pit-wall runoff, or waste water being disposed of in the lake; GWin is groundwater entering the lake; E is evaporation; T is plant transpiration; SWout is surface water exiting the system (including water that is pumped, treated, and discharged to receiving water bodies); GWout is groundwater exiting the lake; and ΔS is the change in storage, i.e., the volume of water in the lake. Terms on the left side of equation are water inputs, whereas terms on the right side (with exception of ΔS) are water outputs. If input is greater than output, then ΔS is positive, and the lake volume will increase (Gammons et al. 2009).
Brixel, B., Caldwell, J. A., Wels, C. 2012. Groundwater Issues in the Design, operation, and closure of tailings, waste rock and heap leach facilities. Robertson GeoConsultants, Vancouver, BC, Canada. http://www.infomine.com/library/publications/docs/Brixel2012.pdf
EPA 2003. EPA and Hardrock Mining: A Source Book for Industry in the Northwest and Alaska. A report on hardrock mining. Appendix A Hydrology.
Gammons, C.H., Harris, L.N., Castro J.M., Cott, P.A., Hanna, B.W. 2009. Creating lakes from open pit mines: processes and considerations – with emphasis on northern environments. Can. Tech. Rep. Fish. Aquat. Sci. 2826: ix + 106 p. http://www.dfo-mpo.gc.ca/Library/337077.pdf
Golder Associates 2011. Guidance Document on Water and Mass Balance Models for the Mining Industry. Report Project Number 1114280024-001-R-Rev0-1000. http://www.env.gov.yk.ca/publications-maps/documents/mine_water_balance.pdf
He, L. & Wilkerson, G.V. 2011. Improved Bankfull Channel Geometry Prediction Using Two-Year Return-Period Discharge1. JAWRA Journal of the American Water Resources Association Volume 47, Issue 6, pages 1298–1316, December 2011
Jorgensen, D.W. & Gardner, T.W. 1987. Infiltration capacity of disturbed soils: Temporal change and lithologic control. Water Resour. Bull., 23:1161-1172.
Karhu, J. 2005. Hydrogeologian perusteet. Luentomateriaali, 7.9.2005. Helsingin yliopisto (in Finnish)
Kokkonen, T., Jakeman, A.J., Koivusalo, H., Norton, J.P. 2011. Computational Methods for Water Resource Assessment: An Exercise Kit, International Environmental Modelling and software Society (IEMSS), Water Resources Engineering, Department of Civil and Environmental Engineering, Helsinki University of Technology, and Integrated Catchment Assessment and Management Centre, Fenner School of Environment and Society, The Australian National University, ISBN 978-88-903574-0-4.
Morton, K.L. & Mekerk, F.A. 1993. A Phased Approach to Mine Dewatering, Mine Water Env., 12 (1–4): 27–33.
NWT 2007. Mine site reclamation guidelines for the Northwest Territories, Indian and Northern Affairs Canada, Yellowknife, NWT, 2007. http://www.aadnc-aandc.gc.ca/DAM/DAM-INTER-NWT/STAGING/texte-text/msr_1320177195268_eng.pdf
Ritter, J.B. 1990. Surface hydrology of drainage basins disturbed by surface mining and reclamation, central Pennsylvania. Ph.D. Thesis, Pennsylvania State University, University Park, PA, 182 pp (unpubl.).
Salonen, V-P., Korkka-Niemi, K., Moreau, J., Rautio, A. 2014. Kaivokset ja vesi – esimerkkinä Hannukaisen hanke. Geologi 66, 8-19. (in Finnish)
USDA 2007. Technical guide to managing groundwater resources. United States Department of Agriculture.http://www.fs.fed.us/biology/resources/pubs/watershed/groundwater/ground_water_technical_guide_fs-881_march2007.pdf
Vehviläinen, B. 1994. The watershed simulation and forecasting system in the national board of water and environment. Publications of the Water and Environment Research Institute. National Board of Waters and the Environment, Finland, No. 17, 3-16.
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.