Stormwater Quality Control: The State of a Young Art

Highway engineers use two general types of controls to protect streams, lakes, and aquifers from development-associated pollution: temporary controls, which are designed to keep rainwater runoff from carrying loads of silt into creeks from denuded hillsides during highway construction; and permanent controls, which are supposed to filter out sediment, metals, nutrients, oil, and grease running off highways and associated development after highways are completed and in use.

Temporary control devices include:

* Silt fences, a permeable fabric strung between steel poles, were used on 23% of the drainage area at six TxDOT construction sites studied by the University of Texas Center for Research in Water Resources (CRWR). The UT researchers filmed torrents of muddy water sweeping past TxDOT silt fences on a creek next to the Outer Loop during major rainstorms. In addition, they found deficiencies in installation and maintenance. The study concluded that even a correctly installed and maintained system of silt fences might remove only 85% of silt during relatively low-intensity rainfalls, resulting in downstream "water quality impacts which would be apparant to even the most casual observer." "Occasional large storm events which would overwhelm the storage capacity of the temporary controls could be expected to produce even more severe water quality impacts," the researchers added.

* Rock berms, or piles of rocks, are intended to slow the flow of runoff, and were used to treat 53% of the drainage area of the study areas. The study found negligible silt removal for the one rock berm evaluated.

* Temporary retention ponds, which TxDOT engineers relied upon heavily to hold silt runoff during the past three years of construction on the US290 freeway. The temporary ponds were the holes dug to be used later for construction of permanent water quality ponds. On several occasions in the past two years, overflows from these ponds dumped loads of sediment into two tributaries of Barton Creek.

Permanent water quality devices include:

* Vegetative controls, or grass planted on slopes to catch draining highway runoff, can filter out significant amounts of sediment, metals, and grease during minor storms, but are less effective for removing nutrients or during heavy rainfalls. And grass swales are ineffective on slopes with an incline of greater than 6%. Another disadvantage is that swales need to be at least 200 feet in length to be effective, which requires more highway right-of-way.

* Dry detention ponds hold rapid rain runoff from paved surfaces, then release it gradually to prevent flooding. When a pond is designed to hold water for six hours or longer, it also allows time for larger particles of silt in the water to settle to the bottom as the pond drains. If rainfall exceeds the pond's capacity, the pond overflows and becomes ineffective. Other problems are high maintenance costs, chronic clogging of inlets, and inability to retain the smaller particles that carry most highway contaminants.

* Sand filtration ponds solve some of the problems of dry detention ponds by providing a second, downstream pond. A bed of sand at the bottom of the second pond filters out many of the smaller, pollution-laden particles that get past the dry detention pond. The City of Austin has been using this method for over 10 years, and has about 1,000 sand filters in place. (In the ponds constructed on South MoPac and the Outer Loop, TxDOT tried substituting a vertical sand filter held up by gravel walls for the usual horizontal bed of sand employed by the city. This eliminated the need for a second pond, and allowed the highway department to squeeze the ponds into existing right-of-way, but, according to UT researcher Michael Barrett, it left the devices performing little better than simple dry detention ponds. TxDOT's four new ponds on US290 use the standard horizontal sand filters.) However, these devices also have many problems, including expensive maintenance, only moderate removal of nutrients, and rapidly declining effectiveness during heavy rainfall. These devices are designed to capture the "first flush" of accumulated gunk that washes off of highways during the first half inch of rainfall. However, as the UT researchers pointed out, contaminants continue to wash from the highway throughout the duration of a rainfall because of the action of car wheels flipping up water and scouring grease from their undercarriages.

* Wet ponds, which are designed to maintain a permanent pool of water, can be more effective at removing pollutants than the dry detention/sand filter pond combination. Microbes in the ponds break down nutrients and dissolved solids that would pass through sand filters. However, wet ponds require a greater right-of-way commitment than other devices. And, like dry ponds, they discharge directly to creeks. This bypassing of the natural, slow percolation of water through the subsoil found in undeveloped areas may be the greatest long-term threat to the aquifer/springs ecosystem.

None of the water quality devices in use in the Austin area can replicate what nature accomplishes on its own. Infiltration basins and trenches, the only devices that mimic the natural recharge of the water table by percolation through the subsoil, are commonly used in sandy soil in Maryland and Florida, but most of them fail within five years due to rapid clogging. Steep slopes and clay soil may make them impractical for use in the Austin area.

For a comprehensive review of the literature on water quality devices, call the Center for Research in Water Resources at 471-0935. -- N.E.