esm223_16_Reading_2_abstracts - Strategies for Offsetting...

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Unformatted text preview: Strategies for Offsetting Seasonal Impacts of Pumping on a Nearby Stream by John Bredehoeft1 and Eloise Kendy2 Abstract ‘3 Ground water pumping from aquifer systems that are hydraulically connected to streams depletes streamflow. The amplitude and timing of stream depletion depend on the stream depletion factor (SDFi) of the pumping wells. which is a function of aquifer hydraulic characteristics and the distance from the wells to the stream. Wells located at different locations. but having the same SDF and the same rate and schedule of pumping. will deplete stream- flow equally. Wells with small SDFi deplete streamilow approximately synchronously with pumping. Wells with large SDFi deplete streamtlow at approximately a constant rate throughout the year. regardless of the pumping schedule. For large values of SDFi. artificial recharge that occurs on a different schedule from pumping can offset streamflow depletion effectively. The requirements are (l t that the pumping and recharge wells both have the same SDF-I and (2) that the annual total quantities of recharge and pumping be equal. At larger SDF-l values. it takes longer for pumping to impact streamflow in a wide aquifer than it does in a narrow aquifer. In basins that are. closed to further withdrawals because streamflow is fully allocated. water—use changes replace new allocations as the source of water for new developments. Ground water recharge can be managed to offset the impacts of new ground water developments. allowing for changes in the timing and source of withdrawals from a basin without injuring existing users or instream flows. Introduction The conjunctive management of ground water and surface water has been practiced in many parts of the world for nearly a century and has been advocated in the professional literature on water resources management for more than half a century (Blotnquist et a]. 2004). As water resources in basins around the world become fully allocated and the basins become institutionally closed to further development. water managers are increasingly re- examining policies ofconjunctive management. Used cre- atively, conjunctive management can enable a change in water use that may allow new development without adversely impacting existing water users. including eco- system needs for instream llows. ‘Corresponding author: Hydrodynamics Group, 127 Toyon Lane, Sausalito, CA 94965: [email protected] 2The Nature Conservancy, Helena. MT 59601; [email protected] theorg Received March 2007. accepted July 2007. Copyright © 2007 The Author(s] Journal compilation © 2007 National Ground WaterAssociation. doi: 10.1111!].1745-6584.2007.DO3B7.X A typical pattern of change in both land and water use is front agricultural to residential and commercial uses. in many parts of the western United States, this change is accompanied by a shift in water use from sur- face to ground water. In other places. traditional agri- cultural water users are shifting from surface water to ground water sources as a means to save labor. Like sur- face water diversion. ground water pumping depletes streamflow. Much of the earlier work on this topic has focused on the total quantities of water diverted from the stream by pumping induced depletion. Although the quantity of water depleted may be the same. the timing of that depletion can vary significantly when withdrawals move from surface to subsurface sour- ces. it is important to consider the tithing of stream depletion in designing mitigation schemes to prevent adversely impacting down stream water users and aquatic ecosystems. In this article. we provide guidelines to water man- agers and hydrogeologists for mitigating the impacts of ground water development that take into account the tim- ing of stream depletion caused by cyclical pumping. This article explores the impact of seasonal pumping and Vol. 46, No. iiGRDUND WATER—January—February 2008 [pages 23—29} 23 Agricultural Pollutant Penetration and Steady State in Thick Aquifers by GJ. Kraft‘, BA. Brownez, WM. DeVitaZ, and DJ. Mechenich2 Abstract The leakage of pollutants from agricultural lands to aquifers has increased greatly. driven by increasing fertil- izer and pesticide use. Because this increase is recent. ground water pollutant concentrations. loads. and exports may also be. increasing as pollutants penetrate more deeply into aquifers. We established in an aquifer profile a ground water recharge and pollutant leakage chronology in an agricultural landscape where 30 m of till blankets a 57-m thick sandstone aquifer. Pollutant concentrations increased from older ground water ( 19631 ill the aquifer base to younger ground waler (1985) at its top. a signal of increasing pollutant leakage. Nitrate-N increased from 0.9 to 13.2 mg/L. implying that leakage increased from 1.9 to [(1.5 kgfhalyear. Nitrate load and export could in- crease. from 130% to 230% before reaching a steady slate in 20 to 40 years. Chloride increases were similar. Pesti- cide residues alachlor ethane sulfonic acid tESA). metolachlor ESA. and atrazine residues partially penetrated the aquifer profile. Their concentrationiage‘date patterns exhibited an initial increase and then a leveling correspond- ing to the timing of product adoption and leveling of demand. Unlike N()_:. projecting pesticide residue steady states is complicated by the phasing in and out of pesticide products over time; for example. neither alachlor nor atrazine is currently used in the area. and newer products. which have not had time to transit to the aquifer. have been adopted. The circumstances that resulted in the lack of a pollutant steady state are not rare: thus. the lack of steady states in agricultural region aquifers may not he uncommon. Introduction The leakage of pollutants from agricultural land— scapes to ground water degrades drinking water resources (Postle et al. 200-1: Nolan et al. 2002: Fan and Steinberg 1996) and exports contaminants that threaten aquatic eco— systems (Rouse et a]. 1999: Johnston et al. 1999: Howe et al. 1998; Hayes et al. 2003). This pollutant leakage (mainly N03 and pesticide residues) increased greatly over the last several decades. driven by increasing [‘ertilv izer and pesticide use (Figure I). particularly over the period of about 1960 to 1990 (Hallbcrg et al. 1989; Bohlke and Denver 1995: Kellogg et al. 2000). Given that lCorrcsponding author: College of Natural Resources. Univer- sity of Wisconsin—Stevens Point, 800 Reserve Street. Stevens Point, WI 54481; [715] 346—2984; fax [715] 346-2965; [email protected] 2College of Natural Resources, University of Wisconsin- Stevens Point, 800 Reserve Street, Stevens Point. WI 54481. Received January 2007, accepted August 2007. Copyright © 2007 The Authorls) Journal compilation @2007 National Ground WatcrAssociation. doi: 10.1111f].1745-6584.2007.003?8.x the increase in pollutant leakage (pollutant mass per land scape areaetime) is a recent phenomenon compared with ' the residence time of ground water in many aquifers. aquifer pollutant loads (lhe pollutant mass per aquifer volume in aquifer storage} may not generally be at a steady state with pollutant leakage in agricultural re- gions. “Steady state" is defined here as the condition in which the mass of a pollutant and its spatial distribution in an aquifer remain more or less constant over time. In the absence of a steady state. pollutants will continue to penetrate more deeply into an aquifer. increasing the aquifer's pollutant load and its pollutant export to surface water. A steady state develops when modern ground water and its accompanying pollutants penetrate an aqui- fer‘s entire thickness or when pollutants in an aquifer degrade at a rate equal to their leakage. Many studies of N03 and pesticide residues ("pollu- tants") have surveyed their presence and concentrations at local to suhcontincntal scales (e.g.. US. EPA 1990: Kolpin et al. 199321. l993h: Postlc et al. 2004) or deter- mined hon pollutant presence and concentration vary with land use and physical setting te.g.. Hamilton and Vol.46,No,1—GROUNO WATERilanuary—Fetvuary 2008 (pages 41—50] 41 ...
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