42 | Green Roads for Water
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
