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.

Water, Water Everywhere, How Much is there to Drink?

All the water that ever was or will be on earth is here right now. More than 97% of the Earth’s water is within the in oceans. The remaining 2.8% is the water within the land masses. The land masses contain all the fresh water on the planet. Of the land surface water, 77% is contained in icecaps and glaciers and for all practical purposes is inaccessible in the short run. The remaining fresh water is stored primarily in the subsurface as ground water with a tiny fraction of a percent of water is stored as rivers and lakes.

The water on earth never rests, it is constantly moving within the hydrologic cycle along various complex pathways and over a wide variety of time scales. Water moves quickly through some pathways -rain falling in summer may return to the atmosphere in a matter of hours or days by evaporation. Water may travel through other pathways for years, decades, centuries, or more—the groundwater stored in the Wasia aquifer in Saudi Arabia fell from the atmosphere as rain thousands of years ago.

Water enters the atmosphere through evaporation and exits as precipitation -rain or snow. Typically, water remains in the atmosphere as vapor for about 10 days and it is this time that allows water to move from the oceans to the land mass. Then condenses and becomes rain, snow, or mist. The pattern of precipitation changes over time and causing or responding to changes in the climate of the planet. A falling raindrop might evaporate, or perhaps be taken up by a blade of grass or other plant. The rain drop might fall on the ground and form a puddle or run off across hard packed soil or pavement. This water will likely evaporate, infiltrate the soil or travel to a stream and ultimately flow to an ocean; at any point along this journey water can evaporate and start again. The average time for water to go from rain to stream flow to oceans ranges between 16 and 26 days. Mankind has interrupted the flow of streams and rivers to the oceans by diverting water for irrigation and building reservoirs, thus slowing or interrupting its flow to the ocean. http://pubs.usgs.gov/circ/2007/1308/pdf/C1308_508.pdf

Not all surface water flows to oceans. Some lakes and wetlands have no surface drainage. They lose water to evaporation and to groundwater. Water moves much more slowly in the subsurface than in the atmosphere or on land surface. Water that infiltrates the soil can remain in the unsaturated zone where it is returned to the atmosphere by evaporation or plant transpiration or the water it can discharge to the surface in a channel becoming surface flow; or it can begin the longer journey- traverse the unsaturated zone and recharge an underlying aquifer. The water that remains in the unsaturated zone typically remains in the subsurface less than a year. Infiltrated water that travels to the saturated zone, and becomes recharge for the aquifer spends much more time in the subsurface. The time that it takes for water to travel through the entire thickness of the unsaturated zone varies tremendously. It can take mere hours to travel through thin unsaturated zones in humid regions to millennia, for thick unsaturated zones in arid regions. The types of soil and rock, the amount of overburden and ground cover and the thickness of the unsaturated zone determines the travel time.

The available supply of fresh water is limited to that naturally renewed by the hydrologic cycle or artificially replenished by the activities of mankind. The recharge rate, the amount of natural replenishment, varies with weather and can exceed water demands during unusually wet periods or fall far below demands during drought periods. Despite conservation the need for water continues to grow planet wide with the growth of human population and the development of emerging economies.