Monday, August 31, 2009

Groundwater Use and Septic Recharge a Green Solution

It is a Common perception that Septic Systems are highly consumptive. However as studies by the USGS, North Carolina Division of Water Quality and the Dutchess County Water & Wastewater Authority have clearly shown, when designed for the correct densities and recharge rates, well (or even surface water) use combined with septic systems is highly sustainable and ecologically sound (Draper, 2006; USGS, 2002). Surface-water resources and groundwater treated in Septic onsite wastewater treatment systems are non-consumptive because they increase base flow into the watershed, and the water can be reused. Properly designed and managed traditional septic systems, alternative septic systems and clustered septic system are an effective method of waste disposal and trod lightly on the earth’s resources. According to the US EPA alternative septic systems, both single family and clustered, exceed the standards for sewage treatment plants and replenish existing groundwater systems, returning clean water to the earth’s water cycle. These alternative onsite systems can be more sustainable to the surrounding ecosystem than sewers and centralized waste treatment and are certainly less expensive for the homeowners in sparsely populated areas. However, the systems need to work properly and alternative systems with multiple tanks, compressors and various parts require consistent maintenance to continue working properly. Remember though, what goes into your septic system goes into the earth. Think carefully about the products you use to clean your house. Paint, solvents, gasoline, insecticides and poisons should never go down your drain. Every chemical you pour down your drain is buried in your yard. In a multitude of ways your yard is part of the earth’s yard.

The 2006 USGS study of water use and recharge in the Atlanta area (before the watering ban) found that average household indoor water use was 200 gallons per day and in the summer months the total water use increased to about 300 gallons per day including outdoor water use. Sustainability should be examined in light of that level of usage. The Dutchess County Water & Wastewater Authority commissioned a study by the Chazen Company at about the same time to better understand County-wide aquifer recharge rates and to provide guidance for setting sustainable development densities specifically related to the use of individual wells and conventional individual septic systems based upon average aquifer recharge. While the quantitative results of the study would apply to the soil types, rainfall and temperature ranges specific to the watershed studied, extrapolations can be made to nearby locations. The weather from New England to the Mid Atlantic to the South becomes warmer and wetter. Currently, average rainfall for New York is 39 inches per year while for Virginia it is more than 45 inches per year. The hydrologic soil groups present in New York are the same groups present in Virginia, but I would guess there is a higher concentration of C/D and D areas. The predominant area of the study, Wappinger Creek is C and C/D soil category. Chestnut Lick, a large creek, behind my house has similar soil hydrologic properties, but the soils on the acres surrounding the house contain a higher proportion of clay. This may be natural or due to the excavation associated with development of the lot and road.

Shallow groundwater flow, or groundwater runoff, intercepts the land surface, feeding springs, and creeks and seeping back into the surface waters as the perennial flow or streams, rivers and other freshwater bodies such as swamps, lakes and ponds. Deep groundwater flow also known as groundwater runout, does not intercept the land surface, flowing instead directly into the ocean. Of all the Earth’s water, only 3% is estimated to be freshwater. Groundwater is estimated to be more than 30% of the freshwater. Precipitation is the source of all groundwater, both shallow and deep. Hydrology is a young science and the modeling of the water cycle is not complete. The recharge rates and water cycle of the shallow groundwater in humid environments is much better modeled and understood than the deep earth sources of groundwater. So, while the entire water cycle is essential to man’s survival, only the shallow cycle will be discussed here.

Aquifer recharge consists of the portion of rain and snow (mostly rain in Virginia) that seeps through the soil to the saturated water zone, the aquifer. Another form of recharge is interflow which is infiltration water that flow along clay and bedrock layers, and roots to reach surface stream without entering the aquifer. Only the aquifer recharge supports wells and septic system dilution, while both recharge and interflow support the surface water supplies. In watersheds with high clay content in the soils a large portion of the rains is lost in runoff creating seasonal streams and high creek and river flow during the spring and fall rainy season. The average daily aquifer recharge (from rain and snow only excluding septic recycling) for Soils C, C/D and D in Prince William Virginia are estimated 326-583 gallons per acre. It is essential in a sustainable system that the groundwater level be maintained with recharge and adequate surface water is supplied to maintain the ecology even during drought years. My property totals more than 10 acres and our total indoor and outdoor household water usage was clocked during the early summer at between 100 and 150 gallons a day. We do not water our garden; trying to plant only what will thrive in the natural environment unaided. Virginia gets plenty of rainfall and it seems silly to plant anything that requires irrigation. Thus, not only is my septic system non-consumptive, the recharge rate vastly exceeds our water usage (and hopefully our neighbors since our water supply is dependent on total demand and recharge of the aquifer).

Though as demonstrated by the USGS studies, septic is a non-consumptive use of water, it is important that the septic system is designed and operated in a way that protects the environment. Whatever goes down the toilet or the drain goes into the earth. (See Septic Systems and the Ecologically Sustainable Life.)The Dutchess County report used nitrate concentrations at half the drinking water level as a proxy to achieve adequate dilution and natural attenuation of all contaminants. Historically, horizontal and vertical setbacks were developed without consideration of the dilution for wastewater components like nitrate, pharmaceutical residue, caffeine and other substances we humans consume, process or produce. The NY Department of Health separation distances were assumed (and these are almost identical to the Virginia setbacks), but the overall regional density of septic systems was examined to ensure that groundwater resources would not be overwhelmed by the total load of contaminants. The density recommendations were developed based on the nitrate concentration in traditional septic wastewater. Nitrate was used as a proxy because all humans produce about 10 pounds of nitrate per year, it does not easily break down and there is a drinking water standard. The target concentration was half the drinking water level to ensure all outcomes are safely below the standard since household size varies tremendously.

The Dutchess County study and the NC study found that overall average density of on-site waste disposal should not exceed one unit per 2-3 acres for an average size house to ensure water quality and recharge in groundwater supplies. The controlling factor in minimum lot size requirements in the northeast appears to be maintaining water quality, not groundwater recharge. Adequate dilution, soil filtration and time are necessary to ensure sustainable water quality. An interesting point is that it is not cost effective to install central water or waste disposal on parcels larger than about a half acre, since the cost of the piping (line connections) between parcels becomes much too high. Clustered or conservation subdivisions can be built, but need to maintain the overall density by maintaining open space. Those who live in dense population areas might want to look to the sustainable ideas of Adam Matthews and Siobhan O’Connor in Good magazine, the water issue, though, I find their idea of a composting toilet in any environment to be really scary from a public health perspective.

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