Recommended practices

natural remediation methods offer a fourth approach to reducing the risk of water contamination from intensively used highways (Wilson 1999). Roadside vegetation, particularly grass strips or vegetated drainage channels, is one example of this approach. These interventions can improve the quality of water in two ways: by absorbing pollutants from water and by separating pollutants from the release of sediment. The effects of vegetation on contaminant removal depend on environmental conditions, the number and type of plants, and the nature and chemical structure of pollutants.

Vegetated channels along roads slow water runoff, trap sediment, and enhance infiltration. They are little artificial wetlands, engineered and planted to slow the flow of stormwater runoff. The goal is to expose the dirty water to plants and soil, which absorb toxic metals, filter out water-clouding sediment, and neutralize noxious germs. According to the United States Department of Agriculture natural Resources Conservation Service,1 if properly installed and maintained, plants and soil have the capacity to do the following:

• Remove 50 percent or more of nutrients and pesticides • Remove 60 percent or more of certain pathogens • Remove 75 percent or more of sediment

Planting grass buffer strips along potential problem road sections can also decrease the effects and costs associated with sediment deposition. The beneficial effects of grass strips in filtering nutrients, pesticides, and sediments from runoff has been demonstrated, for instance, by Morschel, fox, and Bruno (2004). Reduction rates fluctuate from about 50 percent to 95 percent, depending on vegetation type, strip width, upslope inclination and area, and rainfall characteristics. Trials on high-risk road sections suggest that a 12-meter-wide strip combined with a hedge might be sufficient to completely remove sediment deposits from the roadway.

RECOMMENDED PRACTICES

Road-water harvesting for groundwater recharge

The purpose of groundwater recharge is to store water underground in times of surplus for use during times of shortage and high demand. This practice 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.

Roads can be used for groundwater recharge using several techniques (see chapter 4). 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 such as 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, groundwater quality will very

likely be substantially improved compared with initial runoff quality and will have become more suitable for household purposes (hygiene and sanitation) or as stock water, although further treatment may be necessary for it to be used as 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 3.1). 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 compared with surface water storage. There are also indications that the intense recharge of

FIGURE 3.1

Change in groundwater levels in an area with groundwater recharge from road water, Tigray, Ethiopia

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Groundwater level below surface (m) 0

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Jan Feb March April May June July Aug Sept Oct Nov Dec Month

2013 2014 2015 2016 2017 2018

Annual average rainfall (mm) 600

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0 b.

2012 2013 2014 2015 2016 2017 2018

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Source: Kifle et al. 2019. Note: (a) Groundwater fluctuation in Freweign area, Tigray, Ethiopia. The well is located downstream of a borrow pit converted into a percolation pond. The borrow pit was converted into a pond in July 2014, and monitoring was done for the period 2013–2018. Three additional water storage ponds were constructed at the end of 2016 for groundwater recharge purpose. (b) Rainfall distribution for Freweign area for 2012–18. m = meters; mm = millimeters.

groundwater by a large range of measures, including through water harvesting with roads, can improve groundwater quality by diluting natural contaminants. Woldearegay, Tamene, and van Steenbergen (forthcoming) find that total dissolved salt levels decreased over a 15-year period in well-managed catchments in Tigray, ethiopia, from 730 milligrams per liter in 1991 to 534 milligrams per liter in 2016 after the rainy season, as in Abreha we Atsbeha, for instance.

Suspended solids may accumulate on the bottom of the infiltration structures, causing soil clogging. Once this happens, the infiltration process slows, and recharge ultimately stops. The suspended solids can be inorganic (for example, clays, silts, fine sands) or organic (for example, 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 otherwise heavily disturbed. In some cases, soil organisms (rain worms, termites, or sow bugs) perform the function 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 that it does not interfere with the infiltration process. The water table must be at least 0.5 meter below the bottom of the infiltration structure (trench, pond, or other) so that infiltration rates are not constrained by the underlying groundwater. If water pollution is a concern, a greater distance between the percolation or infiltration structure and the groundwater table is recommended. This greater distance provides an adequate unsaturated zone below the basin bottom for natural water treatment, particularly for aerobic processes (that corrode possible pollutants) and virus removal to occur.

The most important parameters for groundwater recharge of relatively unpolluted water (that is, without PAHs; see box 3.1) are suspended solids content, total dissolved solids, bottom of form (total dissolved salt) 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 or 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 caused by 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 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 also have negative consequences. Moist and waterlogged soil around the road presents a risk to the stability of the road body

itself (Pritchard, Hallett, and farewell 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 buildup 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 two meters of the surface, capillary action could 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 intensively 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 in several areas in Australia, they will crystallize at or near the surface (nSW Agriculture 2003). The average expected lifespan of sealed roads is 20 years, and for heavy duty pavements it is 40 years. Salinity can, however, shorten the expected lifespan of roads by accelerating the rate of deterioration. Low damage levels can reduce a road’s lifespan by 10 percent, and severe damage can reduce it by up to 50 percent. In Pakistan and other countries where waterlogging and salinity is a risk, this reduction in road lifespan is an important reason to improve cross drainage around roads and ensure the productive use of this water.

Road-water harvesting for surface storage

Water from culverts and roadside drains can be diverted to surface water storage points, such as cisterns or ponds. In limestone areas, communities in the Republic of Yemen have developed water cisterns along roads (photo 3.1). The water cisterns are filled by rainfall events, though the events are scarce. Apart from storing water, the cisterns double as cold stores. The water from the cisterns is used for livestock watering in dry periods. Roadside cisterns may have reinforced covers to reduce evaporation and to prevent humans and livestock from falling into the tanks.

In the Republic of Yemen, the first runoff after a long dry spell is often not allowed to enter the cistern because this “first flush” water is contaminated and carries too much dust and sediment. Road water collected in the roadside drainage ditch channel is typically managed by two small porter stones installed across the trapezoidal ditch channel. Mud, sand, or a piece of cloth are used to block the “gate” and divert water to the cistern. During the first flushes and later—after the cistern is filled—these temporary checks are removed. Some roadside cisterns include sediment-trapping facilities, using overflow structures and skimming the cleaner top layer of water. When designing cisterns, there are two major issues to consider: type and amount of storage, and contaminant removal.

Boxes 3.1, 3.2, and 3.3 provide examples of water cisterns used for household purposes that benefit many smallholder farmers in India, Brazil, and China, respectively.