The purpose of groundwater recharge is to store water underground in times of surplus for use during times of shortage and high demand. This is particularly useful where rainfall is concentrated in a short period and when there is no need for additional watering. The beauty of groundwater recharge is that subsurface aquifer systems can store large volumes of water at almost no additional cost.
There are several techniques to use roads for groundwater recharge (see Chapter 2). Water from roadside drainage can be diverted to percolation ponds, trenches, and swales (van Steenbergen et al. 2018) or spread over recharge zones. In recharge zones, runoff collected by a road body infiltrates through comparatively porous, unconsolidated, or fractured material like sand, fractured basalt, and old glacier deposits. The recharge zone is situated on top of the receiving water bearing layer or aquifer.
This water can then be extracted with existing or new hand-dug wells or shallow or deep tube wells, depending on the geology and the depth of the groundwater.
Recharge by infiltration takes advantage of the natural treatment processes that occur when water moves through soil. Thus, when groundwater is extracted, its quality will very likely have been improved over the earlier runoff quality. It will have become more suitable for household purposes (hygiene and sanitation) or as stock water, although further treatment may be necessary for drinking water.
Results from groundwater monitoring undertaken in Ethiopia reveal an increase in groundwater levels following the implementation of road-water management techniques for groundwater recharge (Figure 6.1). Infiltration systems designed for groundwater recharge require permeable soils (sandy loams, sands, and gravels) with relatively high infiltration rates. By storing water in aquifers, evaporation losses are reduced as compared to surface water storage. There are also indications that the intense recharge of groundwater by a large range of measures include water harvesting with roads can improve groundwater quality essentially by diluting natural contaminants. Woldearegay et al (forthcoming) found that total dissolved salt levels (TDS) decreased over a fifteen year period in well managed catchments in Tigray, Ethiopia., from 730 mg/liter in 1991 to 534 mg/liter in 2016 in the post rainy season. in Abreha Weatsbeha for instance.
Suspended solids may accumulate on the bottom of the infiltration structures, causing soil clogging. Once this happens, the infiltration process slows down and recharge ultimately stops. The suspended solids can be inorganic (e.g., clays, silts, fine sands) or organic (e.g., algae, bacterial flocks, sludge particles). When particles accumulate at the bottom of banks of infiltration structures, the particles should be removed after rain events or heavily disturbed. In some cases, soil organisms (rain worms, termites, or sow bugs) play this role of disturbing the soil and removing the clogging particles.
An important design principle for groundwater recharge structures is that the groundwater table must be deep enough below the infiltration system so that it does not interfere with the infiltration process. The water table must be at least 0.5 m below the bottom of the infiltration structure (trench, pond, etc.) so that infiltration rates are not constrained by the underlying groundwater. When there is concern about water pollution, a greater distance between the percolation or infiltration structure and the groundwater table is recommended so that there is an adequate unsaturated zone below basin bottom for natural water treatment, particularly for aerobic processes (that corrode possible pollutants) and virus removal to occur.
For relatively unpolluted water (i.e., without PAHs, see box 6.1), the most important parameters for groundwater recharge are suspended solids (SS) content, total dissolved solids Bottom of Form(TDS) content, and the concentrations of main cations such as calcium, magnesium, and sodium. When there are too many suspended solids, it is recommended that sediment/silt traps be installed to avoid clogging. If the water is meant to be extracted for drinking supply, the main water quality parameters to consider are microorganisms, trace-inorganic chemicals, and anthropogenic organic chemicals. Soils generally act as natural filters that reduce the concentration of pollutants due to physical, chemical, and microbiological processes. In these processes, suspended solids are filtered out; biodegradable organic compounds are decomposed; microorganisms are adsorbed, strained out, or die; nitrogen concentrations are reduced by denitrification; synthetic organic compounds are adsorbed and/or biodegraded; and phosphorous, fluoride, and heavy metals are adsorbed, precipitated, or immobilized. The extent to which soil can remove pathogens depends on several factors, including the physical, chemical, and biological characteristics of the soil, the size and nature of the organism, and environmental conditions such as temperature. The largest organisms, such as protozoa and helminths, are removed effectively by filtration unless the soil contains large pores. Bacteria are also filtered, although viruses may be too small to be filtered by most soil pores (National Research Council 1994).
Groundwater recharge can have negative consequences as well. If the soil around the road is moist and waterlogged, this presents a risk to the stability of the road body itself (Pritchard et al., 2015). In the presence of expansive clays in the soil, a change in moisture content can lead to volume changes resulting in loss of pavement shape and cracking of sealed pavements. The combination of soil moisture and traffic (change in pressure) also leads to a build-up of pore pressure within the base that can cause the soil to crack. The movement of heavy traffic across the road pushes water and fine material out through these cracks, making them larger (NSW Agriculture 2003).
If the water table under the road is at the soil surface or within 2 m of the surface, there is a risk that capillary action will draw moisture into the road pavement. When moisture content reaches the plastic limit of one of the pavement layers, the stiffness of the layer may be reduced. Especially in intensely used road sections, the weight of passing traffic will change the shape of the layer, forcing upper layers to bend and stretch over the weakened lower layer.
In arid or warm dry climates, annual evaporation usually exceeds annual rainfall, leading to the upward migration of soil moisture. If soluble salts are present in this moisture, as reported from several areas in Australia, they will crystallise at or near the surface (NSW Agriculture 2003). The average expected lifespan of sealed roads is 20 years and 40 years for heavy duty pavements. Salinity can, however, shorten the expected lifespan of roads by accelerating the rate of deterioration. Low damage levels can reduce road lifespan by 10 percent, while severe damage can reduce it by up to 50 percent. In Pakistan and other countries where water logging and salinity is a risk, this is an important argument to improve cross drainage around roads and ensure the productive use of this water.