El Paso's Experience ΓÇô Converting Wastewater into Drinking Water
In El Paso, groundwater recharge made sense for a number of reasons. The Hueco Bolson aquifer was being depleted by over pumping, groundwater levels were dropping, and there were fears that the region could run out of groundwater early in the 21 st century. El Paso investigated various ways to increase its water supplies including groundwater pumping in New Mexico, importing water from the TransPecos region, and wastewater reuse. Meanwhile, El Paso needed to expand and improve its sewage treatment facilities. Large amounts of water would have been lost to evaporation if traditional lagoons were utilized.
El Paso decided to treat the wastewater to drinking water standards and then inject it underground in a series of 10 wells. The recharge project also allowed El Paso to recapture and reuse its water supply. If El Paso chose to discharge wastewaters into the Rio Grande, those flows would have been unrecoverable.
The recharge system has been operational since 1985. The recharge water eventually flows to a series of production wells which supply drinking water for the City. It is estimated that recharge water is diluted by a ratio of 20-to-1 as it passes through the aquifer. The time it takes the recharged water to flow to the drinking water wells is estimated at 2 to 7 years. The U.S. Geological Survey is now studying whether boron isotopes and computer models could be used to track flow travel times and patterns, and whether levels of trihalomethanes could increase as a result of the program.
Theoretically, the water could be safe enough to be recycled directly into El Paso's water system once it's been subjected to such high levels of treatment. However, State regulations now ban such direct reuse programs. Also, having the water flow through the soil before it's recaptured adds another measure of protection against bacteria, chemicals and other contaminants that may remain after the treatment process. No significant changes in water quality have been reported in the aquifer since the program began.
Roughly 7,850 AF were recharged into the aquifer in 1986. City officials predict that the system will probably be doubled in size within the next 10 years to supply 25% of El Paso's water needs (19,000 AF annually). Costs for the treatment and recharge system totaled roughly $1.55 per 1,000 gallons of finished water in 1988.
The project seems to have helped stabilize groundwater levels in the area. Before the ASR program was begun, water levels were dropping an average of 2 feet per year. Since that time, water level declines at the injection site have averaged only 0.6 feet per year.
Dell City: Merging Floodwaters with Groundwater Recharge
In Dell City, an unusual flooding problem led to studies to determine if stormwaters could be trapped and utilized for groundwater recharge. Dell City is located 90 miles east of El Paso in a valley that is occasionally victimized by disastrous floods. The lack of natural drainage prevents runoff into rivers and streams and results in extremely swift flows and heavy flood losses. At the same time, groundwater levels were falling because of heavy agricultural usage.
To solve the problem, the U.S. Department of Agriculture/ Soil Conservation Service proposed building four flood control dams. Because there are no rivers in the area it was difficult to drain the reservoirs without causing flood damage. The solution was to develop a series of wells immediately downstream of each dam. Floodwaters would flow into the reservoirs during storms and would then flow to the wells, providing much needed recharge and preventing flood damages.
So far, 11 recharge wells have been constructed (Logan, 1990). Designs are now being developed for a conveyance system that will transport the floodwaters from the dams to the wells. When the system is operational in 1991, it could add as much as 6,000 AF of fresh water to regional aquifers each year, depending on the amount of rainfall. This could help stabilize groundwater levels and decrease the salinity of groundwater supplies.
High Plains: Putting Playa Lakes to Work for the Ogallala
The Texas High Plains are covered with more than 17,000 shallow, naturally occurring, depressions called playa lakes. The lakes capture an average of 1.8 to 5.7 million AF of water annually.
The characteristics of individual playas vary tremendously. Some cover only a small area while others can be as large as 200 acres. Most of the playas are 2 to 10 feet deep when full of water. There are an average of one to two playas per square mile in the Southern High Plains.
In the spring and fall, intense thunderstorms frequently fill the playas. However, little infiltration occurs, 50% to 80% of the water is lost to evaporation and winds, and minimal groundwater recharge occurs.
Researchers at the Texas Tech University Water Resources Center have been investigating the amount of water playas naturally recharge and whether recharge could be increased by modifying the playas.
Almost all playa lakes have a clay bottom. However, the clay layer thins out and is often replaced by sandy soils in upper portions of the lakebed. When enough rain falls to raise the water level above the clay bottom and into the sandy areas, significant natural recharge can take place. After the lake levels fall back into the clay zone, little recharge occurs.
The clay layer also presents water quality problems. When clays shrink they crack and briefly provide a direct conduit for contaminants to enter shallow soils. When plants grow in playa lakebeds, their roots can also create openings and cracks.
The Texas Water Resources Institute sponsored studies by Texas Tech scientists to quantify the amount of water available for recharge. Many of the studies involved evaluating the effectiveness of filters to reduce clogging of wells and aquifers, and removing clay from the bottom of the lakes (Urban and others, 1990).
The amount of water lost to evaporation can be as much or more than the amount recharged to the Ogallala in many playa lakes. However, studies at Texas Tech suggest that 60 to 80% of the water now lost to evaporation can be recharged when playas are modified.
The modifications involve installing a textile filter in the lake bottom that is covered with a thin layer of clay and sand to improve the quality and the amount of water being recharged to the aquifer. The thin clay layer is needed to trap suspended solids and filter out other contaminants. The water flows through the filters into pipelines connected to recharge wells and then into the aquifer.
In a demonstration project near Shallowater, the quality of the recharge water was better than native groundwater for all parameters except total suspended solids and chemical oxygen demand (Urban and others, 1988).
Texas Tech scientists have also proposed a large scale project to demonstrate the feasibility of playa lake recharge. The effort would involve assessing the costs and benefits of modifying 8 playas with highly variable characteristics. Hopefully, filters and recharge wells could be developed that would last 7 to 8 years - long enough to make the modifications pay for themselves.
Researchers with the U.S. Department of Agriculture/Agricultural Research Station at Bushland have also found that basins excavated in playa lakes can produce significantly more recharge than unmodified lakes. Work is also going on to divert stormwater runoff into sinkholes and wells drilled into the Blaine Gypsum aquifer which straddles the Texas-Oklahoma border in Hardeman and Collingsworth counties (Runkle and Johnson, 1989).
The implications of widespread recharge projects could be significant. Individual playa lake recharge projects could save enough water to sustain irrigation on 20 to 50 acres without depleting the aquifer.
