Wastewater reuse: What's left behind in soil

The reuse of treated wastewater has become a valuable resource in arid regions around the world, with water being reclaimed for a variety of purposes including irrigation, surface release, and groundwater recharge. But contaminants, such as pharmaceutically active compounds (PhACs) have been identified as potential health problems for wastewater reuse. And understanding the conditions in which PhACs accumulate and degrade over time, may help scientists develop a way to rid the environment of these potentially harmful compounds.

Waste water reuse

This possibility inspired a pair of USDA-ARS scientists from the U.S. Arid-Land Agricultural Research Center (ALARC) to study soils from a groundwater recharge basin in Gilbert, Arizona.  The search by Clinton Williams and J.E.T. McLain targeted four well-known PhACs: ibuprofen, caffeine, carbamazepine, and lincomycin.

Exposure to these PhACs may have serious health consequences, one being antibiotic resistance. Colonies of bacteria exposed to these compounds in nature can develop into resistant strains over time, leading to diseases that require more expensive and toxic medication. PhACs have also been linked to endocrine disruption in humans, affecting hormone glands regulating reproductive growth, metabolism, and other essential body functions. However, the effect on the endocrine system has not been proven.

While several studies in the past have investigated the fate of PhACs in stream and river systems, as well as irrigation practices, little is known about the accumulation of these contaminants in the soil of groundwater recharge systems.

Groundwater recharge occurs naturally as surface water infiltrates through the earth and replenishes groundwater aquifers, filtering the water in the process and elevating the water table. Artificial groundwater recharge using reclaimed sewage effluent is becoming increasingly common in arid environments as surface water supplies dwindle and water tables retreat further underground.

The basin involved in this study utilizes a recharge method known as soil aquifer treatment (SAT). This treatment system involves the infiltration of treated wastewater through a recharge basin and its eventual extraction through nearby recovery wells. During the cycle, treated effluent undergoes a variety of further treatment. Before infiltration even begins, contaminants in the water can be degraded by sunlight and oxidation. Soil life carries out biodegradation, taking in wastes and recycling them into useful nutrients. The vadose zone, an unsaturated area between the land surface and water table, acts as a filter by removing suspended solids, bacteria, and viruses.

In the study, which appears in the September-October issue of the Journal of Environmental Quality, the USDA-ARS duo collected soil samples from a 4 ha Gilbert recharge basin. Over the course of three years, they examined the accumulation rates of PhACs in the samples to determine the long-term sustainability of the SAT system in removing these contaminants.

The anti-inflammatory compound ibuprofen was below detection limits in all samples. Lincomycin, an antibiotic, showed no net accumulation over the three-year study but had significantly higher concentrations closer to the surface than at greater depths. The stimulant caffeine exhibited net accumulation throughout the study, and its greatest concentrations came near the surface as well.

But perhaps the most interesting PhAC studied was the final compound carbamazepine, a drug used to treat epilepsy and bipolar disorders. While it did show accumulation over the course of the study similar to caffeine, it had the lowest concentration of the four compounds at a depth of 0 to 5 cm.

“You can look, and you can find these compounds everywhere at very low concentrations. But I’m mostly interested in what we saw happen with carbamazepine,” says Clinton Williams, an author of this study. “There’s something going on at the surface, and that’s what I want to figure out.”

Understanding why carbamazepine concentrations appear lowest at the soil surface may hold the answers as to what processes are responsible for degrading PhACs leftover from wastewater. Knowing this could allow scientists to increase the sustainability of wastewater reuse for groundwater recharge purposes, a development that may be crucial to meeting future water demands while protecting human health. The question remains as to what consequences, if any, compounds left in soils may have on human health.

View the abstract at https://www.soils.org/publications/jeq/abstracts/41/5/1473

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