Poster Abstracts Conduits, Karst, and Contamination − April 19, 2012 Minnesota Ground Water Association Spring Conference 1 ‐ Establishing a Lysimeter Network in Southeastern Minnesota to Assess Nitrate Transport Through the Root Zone of Agricultural and Non‐agricultural Soils Heidi Breid, Chelsea Hawkridge, and Toby Dogwiler The Driftless Area of southeastern Minnesota includes a variety of competing land uses spanning agriculture, recreation and tourism, and high‐value cold water trout streams. The Driftless Area includes the largest karst terrain in the upper Midwest. Agriculture is a long‐standing foundation of the economy within the region. Understanding and managing the impacts of agricultural practices on sediment transport and water quality is a primary focus of local resource managers. This challenge is made more urgent and critical by the rapid growth of recreation and tourism focused on the trout stream resources of the Driftless Area. Agricultural nutrient management is recognized as a critical water quality concern in southeastern Minnesota. An important question centers on the flux of nitrate through the soil and beyond the root zone. Nitrates leaching past the root zone represent a dual‐edged problem. Once below the root zone the nitrogen is no longer available to the crop and represents a wasted agricultural resource. Furthermore, the nitrate is fated to become a contaminant delivered to the trout streams by shallow subsurface throughflow and groundwater movements expedited by the karst hydrology. To quantify the flux of nitrate through the root zone a lysimeter network has been established in southeastern Minnesota. The lysimeters are primarily deployed in various agricultural cropping and management systems, but as a point of comparison sites representing prairie, forest, residential, and golf courses have also been instrumented. The lysimeters were installed in spring and summer 2011 and are being sampled weekly. The nitrate levels in the preliminary water samples from the lysimeter network range from 0 mg/L to 32 mg/L. Generally, the highest values derive from row crop agricultural land uses and lower concentrations are associated with pasture and non‐agricultural land uses. . As sufficient data is collected it will be possible to understand the relationship between nitrate flux through the root zone and various land uses and agricultural management practices. This will guide recommendations regarding BMP implementation. The preliminary data also suggest that much of the excess nitrate reaching the groundwater, and eventually trout streams, is agriculturally‐derived. 2 ‐ Surface and Groundwater Nitrate Databases For Southeastern Minnesota, USA Greg Brick, E. Calvin Alexander, Jr., Justin Watkins, and James R. Lundy Results of a Minnesota Pollution Control Agency (MPCA) funded collation of nitrate databases for surface and groundwater of the 10 counties of southeastern Minnesota’s (SE MN) karst region are presented. The goal is to identify existing data that can help define time trends and mechanisms of nitrate water pollution. The U.S. Geological Survey’s National Water Information System contains data collected from 1889 to present for the entire nation and is available on the Internet. The MPCA created three successive state‐
wide ambient (background) groundwater monitoring programs, which includes nitrate data, maintained in the EPA’s STORET database: from 1978 to 1990, from 1992 to 1996, and from 2003 to present. In the current program, urban surface and groundwater are sampled by the MPCA while rural waters are sampled by the Minnesota Department of Agriculture. Another MPCA program, Milestone Rivers, has nitrate data from 1953 onwards. The Garvin Brook watershed of SE MN was studied in more detail. The Minnesota Department of Health (MDH) began recording nitrate data from the initial testing required for the installation of new wells in 1974. Eventually these data will be linked to the County Well Index, maintained by the Minnesota Geological Survey, which includes aquifer‐specific information and unique well numbers. MDH also maintains the Minnesota Drinking Water Information System, collecting nitrate data from public water systems. Another MDH effort, since 2008, is the Voluntary Nitrate Monitoring Network, unique to SE MN, which relies on citizen volunteers to sample domestic well waters. The Minnesota Department of Natural Resources’ (DNR) Part B of the County Geologic Atlas series often includes nitrate data, available on the Internet. The DNR’s state fish hatcheries have monitored nitrate concentrations in source springs. Smaller units of government and independent researchers have accumulated much data. The Olmsted County Regional Laboratory has more than 36,000 well‐water analyses from surrounding counties. County nitrate clinics, where individuals can have well‐water tested, are another large source of data. The Metropolitan Council (Twin Cities) collects surface water samples along the Mississippi River. University of Minnesota dissertations also contain much nitrate data. 3 ‐ Characterization of fracture flow and contaminant transport in a siliciclastic bedrock aquifer near a public supply well Christopher A. Gellasch1, Kenneth R. Bradbury2, David J. Hart2, Jean M. Bahr1 1 Department of Geoscience, University of Wisconsin‐Madison 2 Wisconsin Geological and Natural History Survey, University of Wisconsin‐Extension In order to protect public supply wells from a wide range of contaminants, it is imperative to understand physical flow and transport mechanisms in an aquifer system. Although flow through fractures has typically been associated with either crystalline or carbonate rocks, there is growing evidence it is an important component of flow in relatively permeable sandstone formations. The objective of this work is to determine the role that fractures serve in the transport of near‐surface contaminants, such as wastewater from leaking sewers, to public supply wells in a deep bedrock aquifer. The Cambrian aquifer system in Madison, Wisconsin was studied using a combination of geochemical and hydraulic testing in a borehole adjacent to a public supply well. Data suggest that bedrock fractures are important transport pathways from the surface to the deep aquifer. Analyses of water from isolated aquifer intervals show that patterns of elevated wastewater indicators in discrete fractures vary between pumping and non‐pumping conditions. The change in flow field due to pumping may allow contaminants to mobilize rapidly through the fractures and reach other portions of the aquifer. Vertical flow assessment of the borehole revealed several fractures that contributed significantly to downward flow in the borehole. These fractured intervals had hydraulic conductivity values several orders of magnitude higher than non‐fractured intervals. With respect to rapid transport of contaminants, high hydraulic conductivity values of individual fractures make them the most likely preferential flow pathways. The siliciclastic bedrock (mostly porous sandstone) between the fractures may act as a “relative aquitard” (especially in vertical direction) due to hydraulic conductivity values several orders of magnitude lower than adjacent fractures. This study suggests that in a siliciclastic aquifer near a public supply well, fractures may have an important role in the transport of sewer‐derived wastewater contaminants. 4 ‐ Water Resource Assistance to the Comarapa & San Isidro Cooperativas, Santa Cruz Province, Bolivia Jeffrey A. Green, PG jagreen@uwalumni.com Watershed management for potable water, water quantity and quality protection and stream flow management are issues worldwide. In order to deal with these issues, local governments often form water management organizations. In the South American nation of Bolivia, these organizations are called Cooperativas. Two of them are the Cooperativas serving the areas around the towns of Comarapa and San Isidro in the western part of Santa Cruz province. They are in the foothills of the Andes Mountains; the bedrock is well‐cemented sandstone that has been uplifted by the rise of the Andes Mountains over the last 70 million years. The rainfall is in the 12‐16 inches per year range. The San Isidro River and the Comarapa River have cut through the foothills, creating plains that are used for irrigated agriculture. As the ground water in that area is generally slightly saline and unfit for human consumption, the rivers also serve as the source for domestic water supply. The Comarapa River and the San Isidro River both have high colloidal sediment loads that create high turbidity levels. The rivers also have fecal bacteria and receive runoff from farm fields that have been treated with pesticides. Because of these issues, the preferred source of water is pipelines that extend into the source areas for the rivers. The rivers rise in the rain‐soaked cloud forests high in the foothills. Comarapa has already extended their pipeline into the cloud forest while San Isidro is planning to do so. The cloud forest tributaries are high enough in the watershed so that the pollution problems are lessened. The Comarapa water board is in the process of purchasing the land in their water supply catchment area. They are working with the local farmers who have not sold their property in the catchment. These farmers are being paid to not graze cattle or row crops. The water board is helping them convert cropland into peach orchards. The return on peaches is about a factor of 10 greater than the return on row crops. The Comarapa and San Isidro Water Boards would like assistance with stream gauging, gauge station placement, precipitation monitoring and investigation of the hydrology of the springs that form the water sources for the cloud forest headwater streams that are used for potable water supply. Through the U.S organization LATCOM, and its Bolivian counterpart EPLABOL, we are working to assist the Cooperativas with these tasks. On a trip to the area in 2011, we began that process. Equipment and analysis for that work was graciously donated by Rickly Hydrological of Columbus, OH and Minnesota Valley Testing Labs of New Ulm, MN. In order to continue the work, we are returning to the area in October 2012. The focus will be on investigating the cloud forest source for Comarapa, setting stream gauges, measuring irrigation canal flow and working with the local hydrologic technician on instrument calibration. We are also planning on working with local people whose water is not supplied by either cooperativa to assist them with simple water treatment options in order to improve public health. In order to accomplish these tasks, funding for equipment and information about alternative water‐treatment technologies are on‐going needs. 5 ‐ Karst Hydrogeologic Investigation of Trout Brook, Dakota Co. Minnesota Joel T. Groten1 and E. Calvin Alexander Jr.2 1 MSc Candidate, Water Resources Science, University of Minnesota, 1985 Buford Ave., St. Paul, MN 55108, grote051@umn.edu 2 Professor, Earth Sciences Department, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455. alexa001@umn.edu Trout Brook in the Miesville Ravine County Park of Dakota County is the trout stream with the highest nitrate concentration in Southeastern Minnesota’s karst region. Water quality data from 1985 and 1995 (Ron Spong, 1995) and by the Dakota County SWCD (Travis Bistodeau, 2006, 2010) document an increasing level of nitrate in Trout Brook. We have initiated a karst hydrogeologic investigation designed to track the nitrate levels and to increase our understanding of the source and movement of nitrates through Trout Brook. Seventeen springs and seeps have been located in Trout Brook’s main branch and tributaries. We are conducting regular monitoring of major anions in the stream and springs, synoptic stream flow measurements and a dye trace of a sinking stream in the Trout Brook drainage. The initial assumption was the majority of Trout Brook’s base flow was from discrete springs. Synoptic base flow and nitrate measurements show that only 30‐40% of the total flow in Trout Brook is from discrete springs. The rest of the base flow appears to be from distributed groundwater discharge directly into the stream. An ongoing dye trace from Weber Run to LeDuc and Bridgestone Springs has begun to define springsheds for the head water springs. Weber Run is a spring fed steam sieve up valley from the Northeast Branch of Trout Brook. Runoff was captured on 2 March 2012 and showed no detectable nitrate in the runoff from a CRP field but elevated nitrate from an adjacent agricultural field, which illustrates the dominance of agricultural sources of nitrate in Trout Brook. 6 ‐ Central Sands Private Well Network 2011 Nitrate Conditions Summary Kimberly Kaiser, Minnesota Department of Agriculture, Pesticide and Fertilizer Management Division, Fertilizer Unit In the spring of 2011, the Minnesota Department of Agriculture (MDA) began the Central Sands Private Well Monitoring Network. During the first year, MDA coordinated the random sampling of 1,555 private drinking water wells throughout Central Minnesota and samples were analyzed for nitrate nitrogen. Home owners from 14 counties participated in this project which was supported by the Clean Water Fund. Over 88.6 percent of the wells sampled had nitrate nitrogen concentrations less than 3 mg/L, 6.8 percent of the wells ranged from 3‐10 mg/L and 4.6 percent were greater than 10 mg/L of nitrate as nitrogen. Older, shallower wells tended to have a higher percentage of results above 10 mg/L. The data collected in 2011 was used to determine current nitrate nitrogen concentrations, determine areas of concern, and to develop a long‐term trend network. 7 ‐ Pb, Cr, As, and Mn Sediment Distributions in a Constructed Wetland Remediation System, Isanti‐Chisago Landfill, Cambridge, MN Meridith Richmond and Jeff Thole, Macalester College, Saint Paul, MN At the closed municipal Isanti‐Chisago Landfill (Cambridge, MN), a FWS constructed wetland system has been treating leachate seasonally since 1996 in response to a groundwater contamination plume. While the treatment of organic contaminants has been well investigated, the details of metals removal and fate are unclear. This project sought to establish the groundwork for understanding the fate of metals via investigation of metal distribution in system and background sediments and belowground cattail (Typha spp.) tissue. Sediment concentrations of Pb, Cr, As, and Mn were analyzed with minimal sample preparation via Field‐Portable X‐Ray Fluorescence (FPXRF) and via bench‐top Wavelength Dispersive X‐Ray Fluorescence (WDXRF) after more extensive sample preparation. The same metals were analyzed in cattail rhizomes after grinding and pelletization via FPXRF. While existing data suggests that most metals are precipitated quickly at the front end of the system, our findings illustrate that only arsenic fits this behavior. For lead, chromium, and manganese, other mechanisms maintain a metal distribution imbalance between the wetland cells relative to the upstream parts of the system (i.e. settling pond and the soil/rip‐rap border before the cells). In the cells sediments have increased Pb and Cr concentrations (102 – 231 ppm and 59 –
101 ppm, respectively) relative to the preceding sites (5 –11 ppm and 7 – 31 ppm). This is reversed for Mn, which has lower concentrations (466 – 641 ppm) within the cells and higher concentrations (1758 – 31145 ppm) before the cells. Concentrations below detection limits for these metals in cattail rhizomes prevented a complete distribution pattern in plant tissues from being developed. These findings do not support our initial hypothesis of first order reduction processes in metal concentrations along the flow path but instead provide a framework for further mechanism‐specific investigation of these distributions. 8 ‐ Hydrogeological characterization of the Cambrian sandstone aquifer in south‐central Wisconsin Stephen M. Sellwood and Madeline B. Gotkowitz, Wisconsin Geological and Natural History Survey Local officials in Columbia County, Wisconsin requested hydrogeological characterization of the regional groundwater flow system to inform water resources management and to protect the county’s drinking water supply. Groundwater is used to meet all drinking water supply needs in the county. However, nitrate is present at concentrations exceeding the state and federal drinking water standard of 10 milligrams per liter (mg/l) in over 500 of 2,500 domestic water wells sampled. Wells with high nitrate are distributed throughout the region, and the source of the nitrate contamination is most likely land application of fertilizer and industrial waste. Project goals include evaluating the potential for widespread nitrate contamination to impact public supply wells that are completed deep in the sandstone aquifer, and conveying this information so that local officials understand the risk of deep contaminant migration. This project started with compilation of readily available geologic and hydrogeologic data into 1:100,000‐
scale water table and aquifer susceptibility maps. Results from a soil water balance model provided the spatial distribution of groundwater recharge across the region. The recharge estimate will inform construction of a three‐dimensional, steady‐state groundwater flow model. The numerical model is based upon a simplified conceptual model that includes two aquifer units: a shallow, unconsolidated aquifer consisting of glacial and fluvial sediment, and a bedrock aquifer composed primarily of sandstone. Although the numerical model will reflect a simplified conceptual model of the bedrock aquifer, heterogeneity within the undifferentiated Cambrian sandstone aquifer likely affects the transport of dissolved solutes within the bedrock. Naturally occurring radium is an additional concern related to deep aquifer groundwater quality. Borehole geophysical techniques, including acoustic and optical borehole imaging, natural gamma logging, temperature logging, and flow logging, show some clay‐rich facies and fractures in these sandstone formations. Additional characterization within the sandstone will include hydraulic testing and analytical sampling of discrete aquifer intervals using straddle packers, and temperature profiling of aquifers using distributed temperature sensing equipment. These data will inform a more robust conceptual model of flow and transport in the bedrock aquifer, potentially leading to differentiation of hydrogeologic units within the currently undifferentiated Cambrian bedrock. 9 ‐ Hydrologic characteristics of spray zone contaminated soils at the National Crude Oil Spill Fate and Natural Attenuation Research Site Leigh Severson, Undergraduate Research Assistant John L. Nieber, Professor Nick Grewe, Undergraduate Research Assistant Department of Bioproducts and Biosystems Engineering, University of Minnesota Soils will typically readily absorb water that is applied to them. However, chemical substances on soil particles can cause the soils to become water repellent to some degree, meaning that when water is applied to the soil the water is shed off the surface. You can see this sometimes in planting soils (flower pots, flower gardens) when the soil because extremely dry. At the Bemidji oil spill site oil sprayed over a portion of the field site during the pipeline break in 1979. This event contaminated the soils. The oil organics covered soil particles, and has made the particles water repellent to some degree. The contamination has caused the surface to be water repellent, and this repellency is suspected to exert a strong influence on the spatial and temporal pattern of recharge and chemical loading to the shallow groundwater underlying the spray zone. The contamination also detrimentally affects the growth of non‐
woody and woody vegetation within the spray zone area. To evaluate the surface and shallow subsurface hydrology of the spray zone, and thereby provide information for future studies of the deeper unsaturated zone underlying the spray zone, a study was conducted in the summer of 2011 to characterize the water repellency of soil profiles within the zone. To conduct this study a surface grid of 80 points was established within the spray zone area. At the grid locations the water repellency of the surface was characterized insitu using the Water Drop Penetration Time (WDPT) test. The WDPT involves placing a drop of distilled water onto the soil surface and measuring the time required for the drop to infiltrate. The degree of water repellency is then classified based on this time into categories of wettable (<5 s), slightly water repellent (5‐
60 s), strongly water repellent (60‐600 s), severely water repellent (600 –3600 s), and extremely water repellent (>3600 s). Each of these categories of water repellency was represented among the 80 locations where the water repellency was assessed. In fact, about 30% of the locations manifested slight to extreme water repellencies. Additional tests in the laboratory showed that when dried some samples of soils that were water wettable (for insitu moisture content condition) in the field also showed signs of water repellency. This suggests that after any extended period of low precipitation the area of water repellency could be greater than the 30% figure. To assess the variation of water repellency with depth in the soil the WDPT was determined at selected depths down to 40 cm at a subset of the grid locations. Among these points, wherever the soil was water repellent to some degree the soil was also repellent down to a depth of up to six inches. At two of the sites with extreme water repellency at the surface the soil manifested some degree of water repellency even to the maximum depth of 40 cm. The water repellency manifested at the surface was also corroborated by infiltration tests conducted at selected sites. Infiltration was low to zero at water repellent locations, but high at water wettable locations. It was also observed that when the soil was water repellent the infiltration that did occur did not wet the soil uniformly, but rather the water entering followed narrow pathways or preferential pathways. This non‐uniformity of wetting can lead to moisture stress effects on plants, resulting in poorer plant health or even absence of plants. The spray zone site has spotty woody vegetation and non‐woody vegetation. It is thought that this is probably due to the non‐uniform distribution of soil surface wettability and to the non‐uniformity of soil wetting at locations where water repellency is significant. 10 ‐ Characterizing Shallow and Deep Groundwater Flow and Nutrient Flux to Deer and Pokegama Lakes, Grand Rapids, MN Jacob A. Smokovitz1*, William W. Simpkins2, John A. Downing3, and Alan D. Wanamaker4 1 M.S. Candidate ‐ Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011. jasmoko@iastate.edu 2 Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011. bsimp@iastate.edu 3 Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011. downing@iastate.edu 4 Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011. adw@iastate.edu An understanding of groundwater/lake interaction is needed to preserve the water quality of northern Minnesota lakes. A Clean Water Partnership Project was initiated in 2010 at Deer and Pokegama Lakes in Itasca County, MN. We hypothesize that groundwater provides a significant flux of nutrients to the lakes. To test this hypothesis we used groundwater data from private wells, minipiezometers, and seepage meters to calculate the nutrient flux from groundwater to the lakes. Stable isotope and nutrient samples were gathered from groundwater in 32 private wells on Deer and Pokegama Lakes during the summer and winter of 2011 in order to create a time series of isotope values and to quantify nutrients in deep groundwater (>50ft below ground surface). Thirteen seepage meter and minipiezometer sites were also installed along the shoreline in both lakes to characterize the shallow groundwater system. Lake water was sampled monthly, and minipiezometers were sampled biweekly and were analyzed for Total Nitrogen (TN), Nitrate (NO3‐N), Soluble Reactive Phosphorus (SRP), Total Dissolved Phosphorus (TDP), Total Phosphorus (TP) and δ18O and δ2H. Results from Summer 2011 showed upward hydraulic gradients ranging from 0.004 to 0.49 in minipiezometers on the shoreline and seepage rates ranging from 0.06 to 0.79 cm/day. Surface topography around Deer Lake suggests that the deep groundwater system with a meteoric signature flows into the lake from the northeast. However, groundwater from the west, north, and south sides of the lake is enriched with δ18O relative to δ2H on the meteoric water line (mean value: ‐8.07‰ δ18O), suggesting that isotopically enriched lake water (mean value: ‐4.97‰ δ18O) is flowing out of the east side of the lake into the deep aquifer. In contrast, the topographic setting of Pokegama Lake suggests that groundwater flows into the lake from all sides; isotopic data from groundwater demonstrate that the composition of deep groundwater falls on or close to the local meteoric water line (mean value: ‐11.61‰ δ18O) and flows into the lake (mean value: ‐7.03‰ δ18O). Deep groundwater NO3‐N and TDP concentrations ranged from below detection limit (BDL) to 0.3 mg/L and BDL to 68 μg/L, respectively. Minipiezometer NO3‐N and TDP concentrations ranged from BDL to 0.4 mg/L and BDL to 482 μg/L, respectively; average minipiezometer NO3‐N and TDP concentrations were used to calculate nutrient fluxes via the shallow groundwater. Based on seepage rates, contributions via shallow groundwater are 0.06 kg/d TDP and 0.01 kg/d NO3‐N at Deer Lake and 0.73 kg/d TDP and 0.57 kg/d NO3‐N at Pokegama Lake. The results of the nutrient flux analysis will be used to guide the Itasca County Soil and Water Conservation District and the Minnesota Pollution Control Agency in managing the future of these lakes. 11 & 12 ‐ What Goes into a Name? Hydrostratigraphic Naming Conventions for CWI Robert G. Tipping, Minnesota Geological Survey Scott C. Alexander, University of Minnesota Jeremy Rivord, Minnesota Department of Natural Resources Hydrogeologic systems span microscopic to continental scales while varying in permeability by more than a dozen orders of magnitude. In the hydrostratigraphic sense, the name implies porosity and permeability characteristics that make a collection of geologic materials distinct from its neighbors. Our conceptual models of groundwater flow are shaped by the language used to describe and define aquifers and aquitards. A name also provides a means for technical and non‐technical people to talk about the same thing. A late 19th century USGS report discussing nomenclature for geologic formations is applicable to hydrostratigraphic naming: “In every case, the definition should be that which best meets the practical requirements of the geologist in the field and the prospective user of the map: That is to say, each formation should possess such characteristics that is may be recognizable on the ground alike by the geologist and the layman.”1 In Minnesota, aquifer names range from colloquial (i.e. the Buffalo or Bonanza Valley) to stratigraphic (i.e. Jordan). Many aquifers, particularly in glacial sediments, have no name at all. In bedrock systems, where depth from the surface and facies changes can create chameleon‐like aquitardifers2, stratigraphy‐based names can create confusion. As our understanding of the spatial distribution and water bearing characteristics of Minnesota’s geologic materials improves, assigning a name connects this technical knowledge to decision makers who may have no background in groundwater. In this poster session, we would like to solicit your views on assigning aquifer names by presenting technical data on glacial aquifers from the recently completed Benton County Geologic Atlas Part B. Specifically, we would like your suggestions for names that benefit the map user, whoever that person might be. 1 United States Geological Survey, Nomenclature: 10th annual report to the Director, 1888‐1889, Part I – Geology, pp. 63‐79. 2 The term “aquitardifer”, as coined by Tony Runkel, was originally applied to the St. Lawrence Formation stratigraphic unit where it is vertically an aquitard but is an aquifer horizontally. 13 ‐ The effect of vegetative type on shallow and deep recharge as revealed by water balance modeling Mikhail Titov, Graduate Research Assistant John L. Nieber, Professor Department of Bioproducts and Biosystems Engineering, University of Minnesota The observed increases of flow statistics in the Minnesota River Basin has sparked controversy over the possible causes of the increases. One plausible argument put forward is based on the hypothesis that observed increases in precipitation during the past few decades are to be blamed for the increases in flow. An alternative to this hypothesis is the hypothesis that the increases in flows have resulted from changes in land management (e.g., surface and subsurface drainage) and changes in vegetative cover, both being associated with large scale agricultural production. Experimental studies have been conducted in Minnesota and Wisconsin to test the effect of vegetative cover change, and those studies have shown an increase in deep percolations beneath areas in which the vegetative cover has been converted from perennial grasses to row crop production. Since groundwater recharge eventually becomes streamflow, an increase in recharge should also increase streamflow. To test the resilience of vegetative systems to increases in precipitation the SWAP (Soils‐Water‐Atmosphere‐Plant) model was applied to two distinct soil and climatic locations in Minnesota, one location being in the south‐central region (near Waseca) and the other being in the north‐central region (near Grand Rapids). The north‐central region was included in this analysis because it might serve as a reference; it has experienced increased precipitation in recent decades but has not manifested concurrent increases in streamflow. In the south‐central location the vegetative cover types examined included perennial prairie grass, continuous corn, and corn/soybean rotation. The soil types considered included Nicolet clay loam and Clarion loam. In the north‐central region the vegetative cover types included deciduous forest and evergreen forest. The soil types considered included Menagha sandy loam and Graycalm loamy sand. Parameters for each of the vegetative cover types included growing season period, leaf area index, and root distribution (depth and density). Also included was the presence/absence of a litter layer, or duff layer, on the soil surface. Simulations were performed for two periods; 1950‐1979 and 1980‐2009. These two periods showed distinctive increases in streamflow in the Minnesota River, as well as distinct increases in annual precipitation. Results of the simulations showed that the deep‐rooted and long‐season prairie grasses produced significantly less deep percolation (20 mm) than the row cropped vegetative cover (160 mm) for the period 1950‐1979. It was also found that the increase in precipitation observed in the 1980‐2009 period led to increased percolation for both cover types, but the amount of increase was significantly less for the prairie cover (30 mm) than for the row crop cover (110 mm), indicating a greater resilience in behavior for the prairie vegetative cover type. The results for the northern forested region did not show any dramatic difference between the deciduous cover and evergreen cover types. While the deciduous cover type produced more evapotranspiration, the evergreen cover type produced more interception, with the sum of the two components being essentially equal. Also, the increase in precipitation in the 1980‐2008 period did not produce any significant increase in percolation, showing a strong resilience in percolation production behavior. 14 ‐ Understanding Long‐Term Natural Attenuation of Crude Oil in the Subsurface Jared Trost1, Kimberly Boland2, Barbara Bekins3, Isabelle Cozzarelli4, Mary Jo Baedecker4, Robert Eganhouse4, Jeanne Jaeschke4, Melinda Erickson1 1 U.S. Geological Survey, Mounds View, Minnesota, USA 2 Bemidji State University, Bemidji, Minnesota, USA 3 U.S. Geological Survey, Menlo Park, California, USA 4 U.S. Geological Survey, Reston, Virginia, USA Long‐term multi‐disciplinary investigations into the processes that limit the extent of subsurface hydrocarbon contamination have been conducted at a site near Bemidji, Minnesota, USA, where crude oil was spilled in 1979. Long‐term trends in the concentrations of (1) benzene, toluene, ethylbenzene, and xylene (BTEX) in the source oil body and groundwater and (2) sediment Fe (III), a source of electron acceptors critical for hydrocarbon biodegradation, were examined. Groundwater and crude oil samples were collected periodically from 1987 through 2010 and analyzed for BTEX. Sediment samples were collected and analyzed for Fe (III) concentrations from 1993 through 1995 and again from 2006 through 2008. At most locations, benzene concentrations in both the oil body and near‐source groundwater decreased between 1987 and 2010, whereas ethylbenzene concentrations remained consistent over this same time period. Ethylbenzene is less water‐soluble than benzene, and hence a larger fraction remains in the oil despite similar exposure as benzene to recharge and groundwater. The dissolved benzene and ethylbenzene plumes have expanded at a rate of approximately 3 meters per year since 1995, extending to 105 meters down‐gradient from the center of the oil body. Previous research at the site indicated that iron‐
reducing bacteria utilize iron oxy‐hydroxide coatings on the aquifer’s sand and gravel sediments to oxidize the hydrocarbons. Since BTEX is still being supplied by the source oil, the plume continues to expand as this Fe (III) source is consumed. Between 1993 and 2006, Fe (III) decreased by an average of 20 micromoles per gram of sediment in the zone 60 to 120 meters down‐gradient from the oil body. In general, when BTEX compounds remain in the source zone, contaminant plumes are prone to expansion because of depletion of electron acceptors inside the plume and limited mixing with dissolved electron acceptors outside the plume.