Aquifer Recharge, Storage, and Recovery - Southwest Hydrology ...

Water Quality Changes During Subsurface StoragePeter Fox – Arizona State UniversityWhile underground storage ofwater offers many benefits,transformations in waterquality resulting from the recharge ofone type of water into an aquifer ofdifferent composition warrant attention.The primary constituents of concern thatmay be introduced into or form in storagesystems include organics and pathogens;nutrients and inorganics can also causeproblems. These constituents may bebroken down, mitigated, or otherwisechanged through chemical, biological,adsorption, dilution, or filtration processesthat take place in the storage zone. Theextent to which these processes occurdepends on the compositions of the twowater types and the subsurface conditions.Time, Surface Area Are FactorsSome water quality transformations occurrapidly, such as those that result fromchanges in oxidation-reduction (redox)conditions or chemical interactionsbetween the injected water and the aquifer.These reactions may not only changethe water quality but also impact thehydraulic capacity of injection wells.Other transformations, such as thebiodegradation of trace organiccompounds, often occur slowly; sufficienttime is required to achieve the full benefitsof aquifer storage. For7.0example, with enoughtime, natural attenuation6.0processes can improve5.0the quality of storedwater to that approaching 4.0native groundwater.DOC (mg/L)Alluvial aquifers, comprisedmostly of sand and gravel,contain abundant surfacearea, which permits plentyof contact with the watertraveling through it. Thissurface area mediates manybiogeochemical reactions thatcan improve water quality. Suchwater quality transformationsare less likely in fractured and3.02.01.00.024 • May/June 2008 • Southwest Hydrologykarst aquifers where preferential flowthrough cracks and fissures limits contactbetween the water and aquifer material.A select group ofSOCs has beenfound to persist inthe subsurface, andthe list is growing.Flowpaths also affect transformationsduring subsurface storage. Flowpathssurrounding dual-purpose wells—usedfor both injection and recovery—havehighly variable travel times and themost unpredictable effects on waterquality. Subsurface storage systemswith different recharge and recoverypoints can have defined flowpaths andassociated travel times. Such systemshave more consistent and predictablewater quality transformations.Oxygen Matters AlsoMany organics, nutrients, and pathogens ofconcern can be removed in the subsurfacethrough biological mechanisms. Keyfactors that affect biological removalduring subsurface storage include thebiodegradable organic carbon content of0 2000 4000 6000 8000 10000 12000 14000 16000Distance downgradient (feet)Dissolved organic carbon (DOC) concentration is shown as afunction of distance for the Mesa (Arizona) Northwest WaterReclamation Plant groundwater recharge basins. Each 1,000 feetof travel equals around six months of travel time. After severalyears of travel, DOC concentrations are less than 1 milligram perliter, approaching background conditions of the aquifer.the recharge water and redox conditionsof the aquifer. Therefore reverse-osmosistreatedwater, lacking organic carbon,does not support significant biologicalremoval. In fact, trace constituents suchas N-nitrosodimethylamine (NDMA)have been shown not to attenuate duringthe injection of reverse-osmosis-treatedwater, unlike in other recharge systems.Most biological transformations will occurwhether or not oxygen is present, however,some organic compounds require a specificelectron acceptor to degrade. Manychlorinated organic compounds, such astrihalomethanes, degrade faster underconditions where oxygen is low or absent.Finally, recharging waters high in oxygendemand will create anoxic conditions andincrease the potential for the dissolutionof mineral iron and manganese.The Organic PlayersThree types of residual organic materialsundergo transformation: natural organicmatter (NOM), which is present inmost water supplies; soluble microbialproducts (SMPs) formed during thewastewater treatment process from thedecomposition of organic compounds,and synthetic organic compounds (SOCs)added by consumers and also generatedas disinfection byproducts (DBPs) duringwater and wastewater treatment(Barker and Stuckey, 1999).Most waters contain NOM; reclaimedwaters contain a mixture of NOMand SMPs. NOM and SMPsmeasured together as dissolvedorganic carbon have concentrationstypically measured in the milligramsper liter range. The primary concernover NOM and SMPs is theirpotential to form DBPs and stimulatebiological growth in distributionsystems. Concerns over bothhuman and aquatic health effectsare generally associated with SOCsand DBPs, which are measuredindividually at concentrations ofmicrograms or nanograms per liter.