Dry Wells for Stormwater Management: An Evolving Viewpoint
By Chuck Graf, Senior Hydrologist, Arizona Department of
Dry wells are bored holes designed to dispose of stormwater into the subsurface. Dry wells first appeared for stormwater control in the 1930s in then-tiny Phoenix. In the early 1970s, local governments in the Phoenix metropolitan area started requiring onsite retention of stormwater to reduce peak flooding, typically requiring that no ponded water remain after 36 hours. The new ordinances led to rapid expansion of dry well use to drain away stormwater. Now, more than 50,000 dry wells have been drilled in the greater Phoenix area. However, this unconventional onsite stormwater management approach has not caught on elsewhere in Arizona—less than four percent of installed dry wells are located outside of the greater Phoenix area. This may change, though, as stormwater is increasingly viewed as an underutilized resource and dry wells are seen as a means for aquifer recharge rather than simply a stormwater runoff control tool.
Until 1987, no legal framework existed in Arizona for the regulation of dry wells, even though thousands were then in use. In 1987, legislation directed the Arizona Department of Environmental Quality (ADEQ) to license dry well installers and establish a registration program for existing and newly constructed dry wells. The law expressly limited the use of dry wells to the disposal of stormwater. This limitation was intended to prevent disposal of hazardous chemicals into dry wells, which in the past had caused severe groundwater contamination plumes (some of which are still under remediation).
Formal regulation in 1987 effectively sanctioned the use of dry wells and spurred their construction (see figure). By the end of 2014, ADEQ had registered 51,507 dry wells. Dry wells are used for stormwater control and disposal at virtually every new apartment, hotel, office, and commercial complex developed in the greater Phoenix area in the last three decades, and for subdivision and park drainage as well.
The early dry wells in Phoenix were simple dug or drilled holes filled with rocks. Burgeoning dry well use after 1987 prompted design improvements, which allowed solids to settle out and reduced floating material that had clogged earlier designs. Newer dry well designs continue to enhance removal of sediment and floating material, thereby delivering cleaner water to the injection zone. This improves injection performance and extends dry well lifetimes.
The dry well borehole is drilled in alluvial sediments, through any intervening fine-grained and cemented zones, into a permeable layer of clay-free sand, gravel, and cobbles. The permeable layer serves as the injection zone for the stormwater. ADEQ requires at least 10 feet of separation between the bottom of the injection zone and the water table. Because groundwater commonly occurs at great depth in Arizona’s alluvial basins, installers often have considerable leeway to find an exceptionally permeable zone above the water table that maximizes dry well performance while maintaining a much greater separation distance than the 10-foot minimum.
Chandler is an example of a rapidly growing Arizona city where dry wells are intensively used for stormwater management. A 2005 study examined the groundwater recharge impact of the dry wells. The study found that 3,763 dry wells were installed within city boundaries, draining 1,400 acres of stormwater retention basins. The study pegged natural, pre-development recharge at 191 acre-feet of water per year (af/yr). Post-development recharge through the dry wells was estimated at 2,100-3,100 af/yr in an average rain year. Dry year and wet year recharge was estimated at 770 af/yr and 8,700 af/yr, respectively. Clearly, dry wells are a significant, if not the primary, source of groundwater recharge within Phoenix-area cities relying on them for stormwater management.
Although still not common, drywells are increasingly being used for stormwater management in communities in other states, notably in Washington and Oregon, but also in California. Perhaps most interestingly, in the Los Angeles area they are starting to be used in an integrated way—as much for groundwater recharge as for runoff control.
Potential adverse groundwater quality impact is the biggest concern about dry wells. Although the definitive water quality study probably remains to be done, a number of studies, including a 10-year study in Los Angeles conducted by the Bureau of Reclamation and others, found little evidence for groundwater contamination. A 1985 study in Phoenix found that dry wells had a beneficial effect on groundwater quality with respect to major chemical constituents. This finding is not too surprising considering the notoriously poor quality of shallow groundwater in many areas due to urban and former agricultural influences. In the Phoenix study, the total dissolved solids (TDS) in stormwater entering the dry well averaged about one-eighth of the native groundwater sampled from a monitor well. (Native groundwater TDS was almost 1,400 milligrams per liter).
Further research is needed, of course, to better characterize water quality impacts. Improved dry well designs and development of new pollution control strategies and technologies are always beneficial. Operation and maintenance best practices need to be optimized and documented, as lack of maintenance is the main cause of dry well performance decline and failure. Continued use of dry wells is a certainty. Care and vigilance in properly locating, designing, and maintaining dry wells will remain key to protecting groundwater quality as dry wells evolve from their traditional use to dispose of stormwater to their employment to recharge urban aquifers.