Infiltration practices can have unintended consequences for groundwater supplies.

By David Woelkers and Bradley B. Brogren

Over the past few years, a sharp increase in attention has been directed toward the negative impacts of stormwater runoff from urban development and agriculture practices on our nation's surface waters. This attention has also led to an increased awareness of the negative impacts of heightened peak flows as a result of the impervious nature of human development activities.

One result of this awareness has been the increased emphasis on reducing these negative impacts through the use of better site-design principles and other best management practices (BMPs). The emphasis has been to reduce the quantity of surface-water runoff through a variety of infiltration practices. While the use of infiltration practices is an important part of effective stormwater management, there has been a surprising lack of attention by surface-water managers to the resulting impacts on groundwater from these practices and associated contaminant migration. Areas with good infiltration soils also provide excellent pathways for a variety of contaminants. In addition, underground injection controls (UICs), such as dry wells with very little soil filtration, create conduits of potential disaster to drinking-water supplies.

Surface-water managers also need to consider surface source-water impacts when designing stormwater BMPs. In areas where runoff is draining to surface waters that provide sources for municipal drinking water, extra care must be taken to ensure protection. More than half of the nation's drinking water is provided by surface source waters.

To properly evaluate practices for watershed management and protection, surface-water managers need to consider all impacts within the water cycle, including groundwater and source-water impacts from stormwater BMPs.

Groundwater Contamination

Stormwater runoff not only impacts our surface waters but also can have a detrimental effect on groundwater. This is of particular concern for public or private water supplies that use groundwater as a drinking-water source. Degradation of surface water and groundwater occurs as the natural landscape is developed and pollutants are washed from impervious surfaces to these waters. Drainage wells can expedite groundwater contamination by providing a direct conduit to the aquifer. Stormwater seepage basins minimize the impact on groundwater, provided pretreatment is in effect or contaminants are not of concern in the runoff.

Contaminants or pollutants contained in stormwater include sediments, pathogenic (disease-causing) organisms, heavy metals, volatile and synthetic organic contaminants, chlorides, and nutrients. Drainage wells provide an avenue for these pollutants to contaminate groundwater sources. Properly designed infiltration systems can eliminate the impact of many of these contaminants but still present concerns for certain organic and inorganic chemicals, soluble metals and pesticides, chlorides originating from road salt, and nitrates derived from fertilizers.

Contaminants can be released to groundwater in several ways. Contamination can result from surface-water infiltration, direct migration, surface-water recharge, or interaquifer exchange. Most contaminants occur in shallow aquifers, but certain practices, such as drainage wells (commonly referred to as dry or stormwater wells), can lead to deep aquifer contamination. They can also allow certain contaminants that would otherwise be filtered by soils direct access to shallower aquifers.

The most common cause of groundwater contamination is infiltration. As precipitation infiltrates soils and is pulled down by gravitational forces, it dissolves a variety of materials that it contacts. These materials combine with water to form a leachate that transports a variety of inorganic and organic constituents to groundwater aquifers. Once these saturated zones are reached, the constituents spread both horizontally and vertically in the direction of the groundwater flow. If this contamination is within a community's wellhead-protection area, the forces of well pumps will draw these contaminants into the well water and result in potential drinking-water contamination.

Direct migrations through the use of such UICs as dry wells provide the widest range of contaminants that can be transported to groundwater. Open conduits eliminate any soil filtration that exists with infiltration practices and can result in deep aquifer contaminations from the full gamut of surface contaminants. Surface-water managers need to use extreme caution with UIC practices to ensure wellhead protection for drinking-water supplies.

Although other methods of transport can also result in groundwater contamination, it is infiltration practices that surface-water managers and BMP planners need to carefully evaluate for potential groundwater impacts.

Infiltration BMPs

The use of infiltration practices has increasingly been encouraged by surface-water managers because of the obvious negative effects caused by increased surface flows due to urbanization and associated increases in impervious surfaces. This increased surface flow can alter the natural flow capacities of these waters, resulting in degradation of aquatic health and habitat and flooding in areas previously above floodplains.

Impacts on rivers and streams from increased imperviousness in the watershed include streambank erosion, filling and widening of channels, sediment deposition that destroys fish breeding areas and other habitat, and sharp increases in flash flooding downstream. Other receiving waters, such as coastal estuaries, native wetlands, and lakes and ponds, are also adversely affected by the increased flows. In many cases, particularly with native wetlands, small increases in flow can cause dramatic changes in these fragile ecosystems. It is clear that reducing the increased-quantity flows must remain a primary objective of surface-water managers.

Yet in the rush to develop strategies to help control surface flow quantity, there has been a tendency to lose focus on the effects of these measures on groundwater. This tendency contradicts a basic principle of watershed management: The entire hydraulic or water cycle must be taken into account in watershed planning, including the flows of water beneath the surface.

Surface-water managers should learn as much as they can about the characteristics of the groundwater that could be impacted by the use of infiltration practices. Data can be obtained through groundwater maps, well-water records, groundwater-quality test results, and other various studies. Care should also be taken to examine the characteristics of the land use within the watershed that is recharging the various aquifers. Because impacts from sources beyond a particular area can affect the impact from a practice at a particular site, both upstream and downstream planners need to look at the area beyond the project-site borders.

Infiltration Practices

The obvious advantage of infiltration BMPs is that they can reduce the quantity of stormwater runoff from a particular site by directing the water back into the ground. Infiltration techniques should include an effective pretreatment. A variety of infiltration practices are available, including:

• retention basins,
• constructed wetlands and vegetated systems,
• infiltration seepage basins,
• porous pavement,
• infiltration trenches,
• UICs or dry wells,
• under-parking-lot chamber systems.

The most common infiltration systems are retention basins, or wet ponds. Retention basins are still currently considered by regulators to be the system of choice for holding stormwater on-site and reducing peak flows in runoff. These BMPs can be very effective in capturing surface contaminants, such as sediments and heavy metals, in a particulate state.

Retention Systems

Commonly referred to as wet ponds, retention systems can capture runoff and prevent it from entering receiving waters. Although these systems can be excellent BMPs for removing sediment and other pollutants, regular maintenance is essential to maintain a healthy system. Accumulation of fine sediments can result in a substantial loss of infiltration.

Vegetated Systems

Constructed wetlands and other vegetative systems are excellent BMPs for reducing offsite runoff and for preventing many surface contaminants from infiltrating waters. Care should be taken to not use natural wetlands as a BMP, as changes in surface-water quantities and transported contaminants can greatly impact these fragile systems. Natural wetlands can also be direct conduits to aquifers without the benefits of soil filtration. In addition, the use of native plants in vegetated systems enhances their sustainability and reduces the negative side effects of invasive species.

Vegetated systems

• are a very effective BMP for both pollutant removal and runoff storage,
• work best in areas with shallow groundwater levels are ideal,
• are variations of infiltration and filtration systems,
• can filter a variety of pollutants from runoff,
• can be used to reduce the quantity of flow.

In addition,

• natural wetlands should be preserved and not used as a BMP,
• other wetland BMPs include wetland basins and channels,
• use of "natural" systems in site development can significantly cut down on surface runoff.

Other types of infiltration BMPs can also be effective, but more care is necessary to ensure that certain constituents are considered in the runoff analysis. Without the benefits of vegetative uptake, the risk of transporting these constituents to groundwater increases, and pretreatment should be used where higher-risk conditions exist. This is particularly true in wellhead areas.

Pretreatment of infiltration waters can be accomplished through a combination of BMPs. The best method for infiltration pretreatment is the use of a filtration system. By utilizing these systems in areas of concern, planners can substantially reduce the risk of organic and other contaminants from migrating to groundwater.

Filtration Systems

Filtration systems are BMPs that use media to remove particulates from runoff. Filtration systems include sand-filter or other designed systems, including lined settling ponds or constructed wetlands without a direct groundwater connection. In addition, for many applications that use underground chambers or drainage pipes to aboveground infiltration systems, properly designed catch-basin inserts are highly effective for most applications. One system uses a presettling sediment chamber and dual media, resulting in a high reduction of organic constituents that are of a particular concern for groundwater protection. Some catch-basin inserts have optional filter media to absorb oil, grease, and other pollutants of concern.

Here are some key features of filtration systems:

• useful when circumstances limit the use of other types of BMPs;
• work well as pretreatment systems;
• can be designed into a site plan, such as sand-filter systems;
• filter media can include sand, peat, absorbents, or activated carbon;
• should incorporate presettling sediment chambers;
• require regular maintenance and should be accessible for maintenance;
• can involve catch-basin inserts or in-pipe designs;
• should use nonleaching media;
• overflow bypass should be used to prevent flooding;
• useful for urban hot-spot applications;
• cost-effective where land use does not allow other BMP options.

Groundwater Programs

When designing surface-water systems, managers should be aware of various drinking-water protection programs. Groundwater contaminants from stormwater impacting public drinking-water supplies can be controlled through provisions of the federal Safe Drinking Water Act (SDWA), Public Law 93-523, effective December 16, 1974. The SDWA gave USEPA, and subsequently the states, the authority to implement UIC, wellhead-protection (WHP) programs, and source-water assessment programs (SWAPs), which all relate to protecting groundwater sources of drinking water. The UIC programs currently, as a minimum, require inventorying and impose BMPs on Class V drainage wells impacting drinking-water aquifers. WHP programs came into existence under the 1986 amendments to the SDWA and allow the states to impose required or voluntary standards for delineating source-water areas, inventorying potential contaminant sources, and protecting public water-supply source-water areas. Local approved WHP programs typically include a team of local officials and residents who through defined managerial controls work with local government, businesses, and industry to protect the community's source of drinking water.

The 1996 amendments to the SDWA require the states to submit a SWAP for approval that defines the state program to assess water-supply susceptibility to contamination for all public water-supply sources. These assessments include delineating source-water areas, inventorying potential contaminant sources, determining susceptibility, and informing the public. While WHP covered many of the SWAP requirements for communities using groundwater, assessments were fairly new to most communities using surface water. Protection of surface-water sources is being developed through federal and state initiatives for source-water protection.

Conclusion

EPA has directed states to assess drinking-water supplies on a national scale and to develop recommendations for practices to protect our nation's drinking-water supplies. Through an active interrelationship between surface-water and groundwater professionals, we can develop stormwater practices that will not only result in protecting our surface waters but also prevent further degradation of our source waters both above and below the surface. Further information on groundwater issues can be obtained from your state's environmental protection, natural resources, and/or public health agencies. EPA has information available at www.epa.gov, as do the US Geological Survey at www.usgs.gov and the Ground Water Protection Council at www.gwpc.org. 

David Woelkers is director of regulatory compliance with Hydro Compliance Management Inc. in Whitmore Lake, MI. Bradley B. Brogren is a source-water specialist with the Michigan Department of Environmental Quality Drinking Water and Radiological Protection Division. His responsibilities include coordinating the Michigan Source Water Assessment Program.

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