Water: Nonpoint Source Success Stories
South Carolina (Section 319I - 1994)
South Carolina's goal is to maintain and enhance existing water quality and uses by reducing and eliminating nonpoint source pollution through cooperation among local, state, and federal agencies and the public. Demonstrating new and innovative technologies--such as the on-site artificial wetlands--is one way the state is accomplishing its goal. Demonstrations provide a means to educate homeowners about how they can participate in this vital effort.
Artificial Wetlands Solving Domestic Septic Problems
Working with section 319 funds, the South Carolina Department of Health and Environmental Control has found an innovative solution to the problem of septic tank failures in houses built on poorly drained soils.
Houses built on organic soils that are unsuitable for traditional on-site wastewater disposal because of high water tables, typical in the South Carolina low country, are prone to have problems. Particularly during rainy - periods, the high water table impedes soil infiltration, resulting in ponding and runoff of untreated waste. A recent project undertaken by the State Division of On-Site Wastewater Management may have the answer--an innovative wastewater treatment system especially designed for poorly drained soils, called an artificial wetlands wastewater treatment system or rock/plant filter on-site wastewater treatment system. Using $614,192 in 1990 section 319 funds, the project has successfully demonstrated how this best management practice can be used for on-site wastewater disposal. The Rowe home in Hollywood was selected for the project because of severe septic system failures and the homeowners' willingness to try an innovative solution. The home is located in Hollywood Manor, a low-income sub- division in southwestern Charleston County approximately 25 years old. A performance evaluation of South Carolina septic tank systems--a 1987 three-year cooperative project of the University of South Carolina and the State Health Department--identified homes in the development as having significant septic system problems. This is particularly due to the poorly drained St. Johns soil, a clay-rich soil with a shallow seasonal high water table.
The system is modeled on one developed by Dr. B.C. Wolverton, who designed a simple backyard system that purifies septic tank discharges.* These discharges would otherwise leach directly into the soil and percolate down to the groundwater and nearby waterbodies. The rock/plant system consists of stones buried in a shallow ditch and plants rooted in the stones. Partially treated sewage from the septic tank flows through the stone filter, providing moisture and nutrients to the plants. The plants invisibly cleanse the septic tank discharge while adding beauty to the lawn. The only upkeep is harvesting some plants once or twice a year.
In the underground wetland designed for the Rowes, waste material first enters a standard septic tank, where solids are allowed to settle. Wastewater flows by gravity through the rock/plant filter system--a U-shaped, gravel-filled trench lined with heavy plastic to prevent downward movement of the wastewater. Water-loving plants- -such as elephant ears, canna lilies, and pennyworts-- planted in the trench remove wastewater pollutants. Subsurface soil absorption trenches receive effluent from the filter for final treatment and disposal (Figs. 4-2, 4-3). The artificial wetland, measuring approximately 43 by 16 feet, cost $6,800 and was constructed by six members of the State Health Department.
Wastewater carrying excess nitrogen and phosphorus enters waterways in several ways. Towns may flush under-treated sewage into rivers; inadequate septic systems may discharge into streams, lakes, and groundwater supplies; or rain may wash fertilizer off farmland, down gullies, and into streams. Overloads of nutrients entering domestic wells can cause drinking water to become a health hazard. Excess nutrients can also cause murky water, algal blooms, and fish kills. Nutrients age lakes prematurely, transforming crystal clear trout waters into muddy bullhead holes in only a few decades, instead of centuries.
Wetlands actively work to clean up wastewater. Water hyacinths, cattails, reeds, and other wetland plants take up excess nutrients and can rapidly assimilate nitrogen and phosphorus. These marsh plants also absorb heavy metals, radioactive nuclides, and other toxic wastes. While toxic wastes enter human cells and wreak havoc with human chemistry, wetland plants readily absorb toxins. Contaminated plants can then be harvested and safely discarded.
In addition, marsh microorganisms that thrive in plant root systems eat and decompose pollutants. These - microorganisms break down poisonous organic compounds-- such as benzene, toluene, and PCBs--into harmless - elements that the microorganisms and the plants can digest. According to Wolverton, "A well-designed artificial wetland of 1 acre can purify as much water as a natural marsh of 100 acres."
Water samples collected at both ends of the rock/plant filter demonstration project were analyzed for nitrate nitrogen, five-day biological oxygen demand (BOD5), total suspended solids (TSS), and fecal coliform bacteria. - Preliminary results show that the filter effectively removed bacteria and BOD5 (Fig. 4-4). Based on six samples collected between March and July 1992, the system removed 95 percent of the fecal coliform bacteria from the incoming wastewater. On May 13, 1992, a bacteria level at the inlet measured 230,000 per 100 milliliters; on the same day, the bacteria level at the outlet was only 80 bacteria per 100 milliliters--a treatment efficiency of 99.97 percent. Individual treatment efficiencies for BOD5 ranged from 79 to 94 percent, with the average of six samples at 85 percent. TSS levels were reduced by an average of 65 percent. The treatment efficiency for nitrate nitrogen averaged only 10 percent over the six samples. This might be due to the fact that during the sampling period a majority of plants were temporarily killed back by winter frosts and lack of water (the homeowner was away for an - extended period). Since Wolverton's research showed that plants provide most of the nutrient removal capability, during full growth and with proper management nitrogen removal could reach 80 to 90 percent.
A second artificial wetland on similar problem soil has been constructed in Horry County and appears to be functioning properly. The success of the two systems has led state sanitation officials to consider artificial wetlands as appropriate solutions for emergency repair of malfunctioning septic tank systems.
*Dr. Wolverton, a former senior research biologist for NASA who worked on water purification techniques for space exploration, is now an environmental scientist and president of Wolverton Environmental Services in Picayune, Mississippi.