Monday, August 20, 2012

Recharging Groundwater

From John Hopkins University website
“The increasing use of ground water for industrial, municipal, and irrigation supply in the United States has emphasized the need for recharging the ground water in many areas by artificial means. Although the practice of artificial recharge is not widespread in the eastern half of the United States, it has been important in southern California for water conservation and flood control since about 1895.” This statement was written by David K. Todd in 1956 as the opening lines of a report titled: ANNOTATED BIBLIOGRAPHY ON ARTIFICIAL RECHARGE OF GROUND WATER THROUGH 1954.

Clearly, artificial recharge of groundwater is not a new idea and during the second half of the 20th century it became more common to use structures such as basins and pits to increase recharge or infiltration of ground water or injection of surface water into aquifers. Overdrawing the groundwater aquifers in the coastal regions of our country is most easily seen by the intrusion of salt water. Overdrawing a groundwater aquifer can have many negative impacts on the regional hydraulic balance- a reduction in discharge to surface water at some other location, an increase in recharge from surface water,  a loss of storage in the aquifer by falling water table or a loss of an aquifer by salt water intrusion or some combination of these effects. To remain a renewable resource the amount of groundwater removed from an aquifer needs to match the recharge rate. What we consume much be replaced.

As our cities have spread out the development, characterized by pavement, buildings, and other impervious surfaces prevents the infiltration of precipitation that occurred before development while the increased population increases the demand for water. Changing the recharge rate by reducing open areas or increasing water velocity through pavement can change the entire water balance and ecology of a region. In some areas of the country (and world), groundwater currently being used entered the aquifer a millennia ago when the climate in that area was wetter. That water is not being replaced under these climate conditions and the supply may ultimately be exhaused unless artificially recharged. The amount of groundwater removed from an aquifer needs to be sustainable, matching the recharge rate whether that recharge rate is natural or artificially enhanced.

These days recharge of groundwater through spreading basins, pits, and injection or drainage wells is more widely practiced and will likely increase in areas of limited rainfall. There are many challenges to recharging groundwater. The first is geologic. Except for recharge using injection wells directly into an aquifer, artificially recharged water must first move through the unsaturated zone. Characterization of the soils and geology is essential to determining the viability of an artificial recharge project. Areas where ground subsidence caused by excess withdraws of groundwater from the fine-grained compressible confining beds of sediments cannot be recharged. In addition, geological characteristics such as faults with significant offset, folds, and extensive coarse- or fine-grained sedimentary geological units can control both groundwater flow and the fate of water from artificial recharge. Groundwater is not an underground bathtub of water, the site specific geology will determine the ability to recharge the aquifer and where that recharge must take place.

Artificial recharge of a groundwater basin can be used to store surface water when supplies are plentiful as a reservoir for dry periods when water is less available. Storing the water in the underground aquifer is generally considered less environmentally damaging than dam and reservoir construction, and underground storage significantly reduces water loss from evaporation. Recharging an aquifer has lower capital costs than dam and reservoir construction, but require similar distribution networks and pumping costs tend to be higher. Monitoring water availability and allocations and water rights of overlying landowners complicate the water ownership and allocation. In California “The Water Plan” imports several million acre feet of water from northern to southern California each year. A significant portion of the imported water is stored in the groundwater aquifer by using artificial recharge, but artificial recharge is also used to stretch the water supply which could allow introduction of contaminants into the aquifer.

For 30 years Los Angeles County has recycled the water from wastewater treatments plants. This water from both secondary and tertiary treated wastewater is discharged into spreading basins to recharge groundwater. Groundwater recharge can be done by surface spreading or direct injection wells. California guidelines recommend spreading over injection because of concerns about water quality and potential health hazards. It has long been know that soil filtration improves water quality and soil column studies with secondary effluent from wastewater treatment has shown dissolved organic carbon (DOC) removal of 56% for sandy loam, 48% for sand and 44% for silty sand, with most sites removing 48% of DOC by percolation through 20 feet of soil. Greater depths of soil (80 feet) reduced DOC by 92%. This is how septic systems work and how nature filters groundwater. However, prior to discharge, wastewater is heavily chlorinated and subsequently dechlorinated, but because of high DOC concentrations in wastewater, high concentrations of disinfection by products (DBP) are created.

In 1996 the concentration of the precursors of disinfection by products such as total organic halide (TOX) and trihalomethanes (THMs) were studied in the groundwater basin and it was discovered that these precursors of disinfection byproducts in reclaimed water were not removed by percolation through the soil. Total organic halide removal was only 17%. (Fate of Disinfection By-products in the Subsurface by Colleen Rostad, U.S. Geological Survey.) The quantity and type of DBPs, varies not only by water quality and disinfection conditions, but also by properties of the organic molecules that make up the dissolved organic carbon. Studies in Los Angeles County have found that the precursors of disinfection byproducts in reclaimed water are not rapidly removed by soil percolation. As our need to recycle water expands we are potentially introducing TOX and THMs and many other contaminants into our groundwater aquifers. As coastal cities need to recycle more water and use it to recharge the groundwater aquifer to maintain the supply of available water, we need to better understand what contaminants (and emerging contaminants) are carried in the wastewater and survive soil percolation. The groundwater aquifer serves to dilute the wastewater contaminants that survive soil percolation, but we need to be honest and informed about what we are putting into or leaving in what is ultimately our drinking water supply. An interesting note is that research at Johns Hopkins University seems to indicate that groundwater recharge using soil filtration of wastewater treatment plant effluents may be an effective method of removing trace Pharmaceuticals and personal care products from the water. So recharging groundwater may be preferred over releasing effluent to rivers for downstream reuse.

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