Groundwater recharge

Water balance

Groundwater recharge or deep drainage or deep percolation is a hydrologic process where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface. Recharge occurs both naturally (through the water cycle) and through anthropogenic processes (i.e., "artificial groundwater recharge"), where rainwater and or reclaimed water is routed to the subsurface.

Processes

Groundwater is recharged naturally by rain and snow melt and to a smaller extent by surface water (rivers and lakes). Recharge may be impeded somewhat by human activities including paving, development, or logging. These activities can result in loss of topsoil resulting in reduced water infiltration, enhanced surface runoff and reduction in recharge. Use of groundwaters, especially for irrigation, may also lower the water tables. Groundwater recharge is an important process for sustainable groundwater management, since the volume-rate abstracted from an aquifer in the long term should be less than or equal to the volume-rate that is recharged.

Recharge can help move excess salts that accumulate in the root zone to deeper soil layers, or into the groundwater system. Tree roots increase water saturation into groundwater reducing water runoff.[1] Flooding temporarily increases river bed permeability by moving clay soils downstream, and this increases aquifer recharge.[2]

Artificial groundwater recharge is becoming increasingly important in India, where over-pumping of groundwater by farmers has led to underground resources becoming depleted. In 2007, on the recommendations of the International Water Management Institute, the Indian government allocated ₹1,800 crore (equivalent to ₹38 billion or US$590 million in 2016) to fund dug-well recharge projects (a dug-well is a wide, shallow well, often lined with concrete) in 100 districts within seven states where water stored in hard-rock aquifers had been over-exploited. Another environmental issue is the disposal of waste through the water flux such as dairy farms, industrial, and urban runoff.

Wetlands

Wetlands help maintain the level of the water table and exert control on the hydraulic head (O'Brien 1988; Winter 1988). This provides force for groundwater recharge and discharge to other waters as well. The extent of groundwater recharge by a wetland is dependent upon soil, vegetation, site, perimeter to volume ratio, and water table gradient (Carter and Novitzki 1988; Weller 1981). Groundwater recharge occurs through mineral soils found primarily around the edges of wetlands (Verry and Timmons 1982) The soil under most wetlands is relatively impermeable. A high perimeter to volume ratio, such as in small wetlands, means that the surface area through which water can infiltrate into the groundwater is high (Weller 1981). Groundwater recharge is typical in small wetlands such as prairie potholes, which can contribute significantly to recharge of regional groundwater resources (Weller 1981). Researchers have discovered groundwater recharge of up to 20% of wetland volume per season (Weller 1981).

Depression-focused recharge

If water falls uniformly over a field such that field capacity of the soil is not exceeded, then negligible water percolates to groundwater. If instead water puddles in low-lying areas, the same water volume concentrated over a smaller area may exceed field capacity resulting in water that percolates down to recharge groundwater. The larger the relative contributing runoff area is, the more focused infiltration is. The recurring process of water that falls relatively uniformly over an area, flowing to groundwater selectively under surface depressions is depression focused recharge. Water tables rise under such depressions.

Depression pressure

Depression focused groundwater recharge can be very important in arid regions. More rain events are capable of contributing to groundwater supply.

Depression focused groundwater recharge also profoundly effects contaminant transport into groundwater. This is of great concern in regions with karst geological formations because water can eventually dissolve tunnels all the way to aquifers, or otherwise disconnected streams. This extreme form of preferential flow, accelerates the transport of contaminants and the erosion of such tunnels. In this way depressions intended to trap runoff water—before it flows to vulnerable water resources—can connect underground over time. Cavitation of surfaces above into the tunnels, results in potholes or caves.

Deeper ponding exerts pressure that forces water into the ground faster. Faster flow dislodges contaminants otherwise adsorbed on soil and carries them along. This can carry pollution directly to the raised water table below and into the groundwater supply. Thus the quality of water collecting in infiltration basins is of special concern.

Pollution

Pollution in stormwater runoff collects in retention basins. Concentrating degradable contaminants can accelerate biodegradation. However, where and when water tables are high this affects appropriate design of detention ponds, retention ponds and rain gardens.

Estimation methods

Rates of groundwater recharge are difficult to quantify since other related processes, such as evaporation, transpiration (or evapotranspiration) and infiltration processes must first be measured or estimated to determine the balance.

Physical

Physical methods use the principles of soil physics to estimate recharge. The direct physical methods are those that attempt to actually measure the volume of water passing below the root zone. Indirect physical methods rely on the measurement or estimation of soil physical parameters, which along with soil physical principles, can be used to estimate the potential or actual recharge. After months without rain the level of the rivers under humid climate is low and represents solely drained groundwater. Thus, the recharge can be calculated from this base flow if the catchment area is known.

Chemical

Chemical methods use the presence of relatively inert water-soluble substances, such as an isotopic tracer or chloride,[3] moving through the soil, as deep drainage occurs.

Numerical models

Recharge can be estimated using numerical methods, using such codes as Hydrologic Evaluation of Landfill Performance, UNSAT-H, SHAW, WEAP, and MIKE SHE. The 1D-program HYDRUS1D is available online. The codes generally use climate and soil data to arrive at a recharge estimate and use the Richards equation in some form to model groundwater flow in the vadose zone.

Adverse factors

See also

References

  1. ↑ "Urban Trees Enhance Water Infiltration". Fisher, Madeline. The American Society of Agronomy. November 17, 2008. Retrieved October 31, 2012.
  2. ↑ "Major floods recharge aquifers". University of New South Wales Science. January 24, 2011. Retrieved October 31, 2012.
  3. ↑ Allison, G.B.; Hughes, M.W. (1978). "The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer". Australian Journal of Soil Research. 16 (2): 181–195. doi:10.1071/SR9780181.
  • Allison, G.B.; Gee, G.W.; Tyler, S.W. (1994). "Vadose-zone techniques for estimating groundwater recharge in arid and semiarid regions". Soil Science Society of America Journal. 58: 6–14. OSTI 7113326. doi:10.2136/sssaj1994.03615995005800010002x. 
  • Bond, W.J. (1998). Soil Physical Methods for Estimating Recharge. Melbourne: CSIRO Publishing. 

Further reading

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