Pharmaceuticals and Endocrine Disrupting Compounds:Implications for Ground Water Replenishment withRecycled Water

by Mark J. Benotti1 and Shane A. Snyder2

1Applied Research and Development Center, SouthernNevada Water Authority, P.O. Box 99954, Las Vegas,NV 89193-9954; (702) 856-3684; fax: (702) 856-3647;mark.benotti@snwa.com2Applied Research and Development Center, SouthernNevada Water Authority, P.O. Box 99954, Las Vegas,NV 89193-9954; (702) 856-3684; fax: (702) 856-3647;shane.snyder@snwa.com

Increasing human population has put unsustainabledemands on fresh water resources, particularly in aridareas of the world. One piece of evidence is the extensiveground water depletion that has occurred in areas due tothe combination of population growth and urbanization.Increased water withdrawal leads to ground water deple-tion when withdrawal rates outpace replenishment rates.Ground water depletion is exacerbated by urbanization, aspaved surfaces divert water that would otherwise percolateinto aquifers. Ground water replenishment with recycledwater is an important and necessary practice and offersseveral beneficial outcomes. First and foremost, its usehelps mitigate ground water depletion and the adverseeffects that can accompany it, such as land subsidenceand sea water intrusion in coastal environments. Second,it maintains water within the watershed or basin (i.e., thewater is not lost to surface water outflow from the water-shed or discharge to the ocean). Third, it can provide asignificant cost savings over alternative water sources.

There are, however, concerns over the quality of therecycled water used in ground water replenishment sys-tems. Waste water–borne pathogens, nutrients, and bio-logical oxygen demand are the most significant concernsfor waste water utilities, as these parameters are mostcommonly monitored in receiving waters. Recently, thesetrace-level waste water–derived organic contaminants have

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received a lot of scrutiny as low concentrations of phar-maceuticals and endocrine disrupting compounds (EDCs)have been linked to adverse toxicological effects on aquaticspecies (Snyder et al. 2004). Thus, their presence in recy-cled water warrants better understanding of their behaviorduring ground water replenishment practices.

Generally, in more highly treated waste water, phar-maceutical and EDC concentrations in the effluent arelower. Most commonly, recycled waste water is treatedwith secondary (biological) treatment and disinfectedprior to replenishment. Secondary treatment significantlyreduces concentrations of some pharmaceuticals andEDCs; constituent removal efficiencies generally correlatewith the solids retention time (Clara et al. 2005). Chem-ical disinfection also reduces the concentrations of somecompounds via chemical oxidation. Despite these pro-cesses, some pharmaceuticals and EDCs persist in treatedeffluent at parts-per-trillion (ng/L) to low parts-per-billion(μg/L) concentrations. Advanced treatment technolo-gies, particularly nanofiltration, reverse osmosis (RO),membrane bioreactors (MBRs), and advanced oxidationprocesses (AOP), are more effective at reducing pharma-ceutical and EDC concentrations in waste water effluent(Clara et al. 2005; Huber et al. 2003; Snyder et al. 2007).Following treatment, the treated water is injected or infil-trated into the aquifer for ground water replenishment.

Figure 1 depicts the most common methods by whichground water is replenished with recycled water, eitherthrough direct injection (a) or discharge to an infiltrationbasin (b). For facilities using direct injection, the recycledwater is pumped directly into the aquifer. If the wateris later pumped from the same well through which itwas injected, it is referred to as an aquifer storage andrecovery system. If the water is injected into a differentwell from which it is withdrawn, it is referred to as anaquifer storage transfer and recovery system. For facilitiesusing infiltration basins, the water is discharged to aninfiltration (or recharge) pond, where it slowly percolatesthrough the vadose zone and then reaches ground water.As water passes through the vadose zone, nutrients andpathogens may be removed. This process is referred to assoil-aquifer treatment (SAT).

Many factors influence the subsurface fate and trans-port of pharmaceuticals and EDCs. Hydrogeological con-ditions such as depth to ground water, sediment porosityand permeability, and ground water flow rates help deter-mine the extent of the recycled water plume in the aquifer.Additionally, biophysiochemical properties such as advec-tion, diffusion, adsorption, and microbial transformationcontrol migration of pharmaceuticals and EDCs within therecycled water plume.

Adsorption is a major process controlling the fate andtransport of pharmaceuticals and EDCs in the subsurface(Snyder et al. 2004). Compounds that strongly adsorb toa solid will not migrate long distances after they enter thesubsurface. Conversely, compounds that do not adsorbstrongly have the potential to migrate conservatively withthe recycled water plume if they are not removed bymicrobial degradation. A comprehensive understanding of

Vadose (Unsaturated)Zone

Aquifer

TreatmentFacility

Direction of Ground Water Flow

Recycled WaterPlume

(a)ASR system: water is withdrawnfrom the same injection well

ASTR system: water is withdrawnfrom another groundwater well

Direction of Ground Water Flow

Vadose (Unsaturated)Zone

Aquifer

TreatmentFacility

InfiltrationBasin

Recycled WaterPlume

(b)

SAT treatment: nutrients/pathogens removed

Figure 1. Methods for ground water replenishment withrecycled water; direct injection (a), or discharge to aninfiltration basin (b).

pharmaceutical and EDC adsorption in the subsurface isdifficult because this class of contaminants is composedof many individual compounds that have a wide rangeof structures/functional groups that give them markedlydifferent physiochemical properties. For example, phar-maceuticals and EDCs have a wide range of log Kow

values; therefore, they have a wide range of affinities forthe solid surface. The log Kow of caffeine is −0.07, whichsuggests it would remain largely dissolved in the subsur-face. Whereas the log Kow of 17β-estradiol is 4.01, whichsuggests it is particle reactive and would be primarilyassociated with the soils. In addition, although log Kow

values may provide a good first approximation of adsorp-tion, there are other factors that may affect adsorption,

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such as the concentration of total or dissolved organic car-bon, the pH of the aquifer, and the pKa of the contaminantas well as the aquifer mineralogy.

Pharmaceuticals and EDCs may also be removedin the subsurface by microbial transformation. The termmicrobial transformation is ambiguous as it refers tothe biological transformation of organic compounds byone of the two pathways. Microbes can oxidize organiccompounds and use the carbon as an energy source viadirect metabolism. Alternatively, microbes can transformcompounds by cometabolism, a process by which theoxidation of the compound does not produce any energyfor the organism. Relatively little is known about eitherprocess in relation to these types of contaminants. Someamount of direct metabolism is needed to maintain themicrobial community within the recycled water plume.A significant amount of cometabolism is also likely tooccur, given that rates of cometabolism can be enhancedwith the presence of structurally similar compounds.

Given the complexity of the organic matter presentin the recycled water, it is plausible that certain frac-tions or functional groups of nonorganic matter can serveto induce cometabolism of pharmaceuticals and EDCs.Given the wide range of structures and functional groups,pharmaceuticals and EDCs will likely exhibit a wide rangeof microbial transformation rates in the subsurface. Stud-ies with estuarine surface water have shown that microbescan readily transform many pharmaceuticals and EDCs,such as acetaminophen. Others, like carbamazepine, wereresistant to microbial transformation (Benotti and Brow-nawell 2009; Snyder et al. 2004).

Information pertaining to pharmaceutical and EDCoccurrence, fate, and transport in ground water is scarcecompared with the state of knowledge of these compoundsin surface water. In one comprehensive study, Dreweset al. (2002) investigated the occurrence, fate, and trans-port of pharmaceuticals during indirect potable reuse atseveral different facilities in the United States. At onefacility, where RO was used, concentrations of all tar-geted pharmaceuticals were below detection limits in theRO effluent. In this case, pharmaceuticals and EDCs havelittle bearing on ground water quality as they are effec-tively removed. At another facility using only secondarytreatment and disinfection before the water is dischargedto ground water, the authors measured parts-per-trillion(ng/L) and low parts-per-billion (μg/L) concentrations of8 of the 13 compounds in the effluent. However, six ofthe eight compounds measured in secondary effluent didnot persist in the aquifer. Carbamazepine and primidonewere the most persistent in ground water and exhibitedlittle removal during subsurface travel times of more than6 years.

It is understandable that utilities considering recy-cled waste water for ground water replenishment maybe concerned over the implications posed by pharma-ceuticals and EDCs, particularly if the recycled water isused for indirect potable reuse. A story by the AssociatedPress that documented the presence of these compoundsin drinking water in the United States (Donn et al. 2008)

received a great deal of public attention and promptedthe U.S. Senate Committee on Environment and PublicWorks to host a hearing on the issue in April 2008. Ifutilities and regulators still harbor concerns over the pres-ence of pharmaceuticals and EDCs resulting from groundwater recharge of recycled water, the best approach maybe to implement some form of advanced treatment suchas RO or an AOP. These processes will drastically reducethe concentrations of most pharmaceuticals and EDCsin a waste water stream; however, they are expensiveand would likely result in higher costs for ratepayers.Additionally, they are often energy intensive and meritconsideration with respect to the “carbon footprint” theywould leave. It is also important to recognize that theseprocesses do not make the problem go away. An ROsystem will create a brine stream, which contains higherconcentrations of pharmaceuticals and EDCs that requiredisposal. Although an AOP uses strong chemical oxi-dants (usually hydroxyl radicals) to chemically react withthese compounds, this process creates a wide range oflargely unidentified transformation products that are thenrecharged with the recycled water.

In short, the presence of pharmaceuticals and EDCsshould not deter ground water recharge with recycledwater. Of the compounds investigated in the subsurface,only carbamazepine and primidone have been shown topersist for long periods. Additionally, there are treatmenttechnologies that may be used prior to recharge thatwould significantly reduce the concentrations of theseconstituents.

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