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IDENTIFIYING PREFERENTIAL FLOWPATHS IN FRACT DOMINATED AQUIFERS: IDENTIFIYING PREFERENTIAL FLOWPATHS IN FRACTURE FLOW THE EASTER RIVER PLAIN AQUIFER CASE DOMINATED AQUIFERS: THE EASTER SNAKE RIVERSNAKE PLAIN AQUIFER Carlos D. Soto-López ([email protected]) CASE STUDY November 21 2006 Carlos D. Soto-López ([email protected]) Location: November 2006 Location: Eastern Snake River Plain Aquifer (ESRiPA) in Idaho, United States o 113W) (Figure 1) Eastern Snake River Plain Aquifer (ESRiPA) in Idaho, United States of America (44°N, 113W) (Figure 1) Main problem illustrated: How to use radiogenic, radioactive and stable isotope tracers together with major element concentrations to identify preferential flow pathways in aquifers? Summary: In highly fractured and heterogeneous aquifers (eg: basalt and karst aquifers) preferential flow is one of the primary groundwater flow mechanisms because they act as a major conduit for water, dissolved matter and contaminants. Radiogenic isotope tracers can provide a unique “x-ray” photograph of the physical and chemical processes occurring in the aquifer that is not provided by the elemental solute concentration data alone. Isotopic studies can provide information about mixing of groundwater, flow patterns, recharge rates and locations, together with rates and types of rock-water interaction. In general, the addition of isotopic measurements will give you a clearer idea of the chemical and physical processes than do other chemical parameters alone. Main problem How to radioactive an tracers togeth element co identify pr pathways in aq Summary: In highly heterogeneous basalt and preferential flo primary gro mechanisms b a major con dissolved contaminants. Radiogenic is provide a photograph of chemical proc the aquifer tha by the e concentration Isotopic stud Figure 1. Location of the Eastern Snake River Plain Aquifer in information a Idaho, USA. groundwater, flow patterns, recharge rates and locations, together with rock-water interaction. In general, the addition of isotopic measureme Tracers Used: clearer idea of the chemical and physical processes than do other ch 87 Sr/86Sr, 234U/238U, Tritium (3H), 14C, Stable Isotopes of alone. Oxygen, Hydrogen, Carbon and Sulfur (Sulfate). People affected environmental, ecological impacts: Basalt and Karst dominated aquifers cover large parts of the world and often those aquifers are the primary or sole provider of potable water for the cities and countries that overlie them. Having an understanding of the location and extent of preferential flow pathways is of critical importance to the assessment of groundwater resources, underground disposal sites and prediction of the movement of pollutants in these fractured dominated aquifers. Basalt and Karst dominated aquifers cover large parts of the world and often those aquifers are the primary or sole provider of potable water for the cities and countries that overlie them. Having an understanding of the location and extent of preferential flow pathways is of critical importance to the assessment of groundwater resources, underground disposal This specific case study is focused sites and prediction of the on the Eastern Snake River Plain movement of pollutants Aquifer in in the State of Idaho, these fractured USA anddominated more specifically on aquifers. the Idaho National Engineering and Environmental Laboratory This specific case study is (INEEL)focused region (Figure 1 andEastern on the Figure 2). Where in the 1950 Plain Snake River low levelAquifer radioactiveinandthe nonState of radioactive waste was disposed Idaho, USA and bymore means ofspecifically injections wells the Idaho ontothe National Engineering aquifer. These contaminants are and Environmental moving down gradient from the INEEL Laboratory site taking advantage(INEEL) of region (Figure 1 one and preferential flow. This aquifer is Figure 2). Where in of the largest basalt aquifers in the the low source level USA and1950 it primary for recharge. Regional ground water flow is from the northeast about 67is the percent of the total radioactive and nondrinking and agricultural water in to theradioactive southwest. Finally waste wasgroundwater discharges in a series of large springs between southern Idaho. Twin Falls and King Hill (Figure 3). disposed by means of Figure 2. Snake River Plain Aquifer (ESRiPA) and watersheds that recharge it. regional groundwater flow direction is given by arrows. Also the boundaries injections wells to the Approximate of the Idaho National Engineering and Enviromental Laboratory (INEEL) are shown. Hydrogeological setting: aquifer. These contaminants are moving down gradient from the INEEL site taking The ESRiPA covers an of advantage of area preferential flow. This aquifer is one of the largest basalt aquifers in the 2 approximately 25,000 kmthe and and itconsists is primary The USA ESRiPA of source for drinking and agricultural water in southern Idaho. 3 stores 1.2 to 2.5thin x 1012 m (1 to 2of multiple flows billion acre-feet) and of water.younger Recharge Pliocene Hydrogeological setting: occurs along the Snake River and basaltic rock interbedded its tributaries on the northeastern with unconsolidated 2 12 3 Theedges ESRiPA and eastern of the covers aquifer. an area of approximately 25,000 km and stores 1.2 to 2.5 x 10 m alluvial deposits (1 toaccounts 2 billion Precipitation for 10acre-feet) of water. Recharge occurs along the Snake River and its underlain by Miocene the from northeastern and eastern edges of the aquifer. Precipitation accounts for percent, tributaries while surfaceon water basaltic-rock aquifers and 10 percent, while surface water from streams, diversion and irrigation of land comprises streams, diversion and irrigation of in some places underlain land comprises about 67 percent of silicic volcanic theby total recharge. Regional rocks. ground In flow general aquifer tois water is fromthe the northeast unconfined but thean southwest. Finallysystem groundwater in discharges insome a series of largeplaces springs Figure 3. Large springs that issue from the north wall of the Snake River Canyon unfractured basalt and between Twin Falls and King Hill near Twin Falls, Idaho, discharge thousands of gallons of water per minute. interbedded clay layers (Figure 3). cause confining The ESRiPA consists of multiple conditions. Permeability thin flows of Pliocene and younger basaltic rock interbedded with unconsolidated alluvial deposits underlain by on the younger of the aquifer is by highly variable dueIntogeneral the different Miocene basaltic-rock upper aquifers layers and in some places underlain silicic volcanic rocks. the aquifer kinds is an of fracturessystem and but features pillow columnarclay basalt lava flows) unconfined in some(lava places tubes, unfractured basaltlava, and interbedded layers and causeindividual confining conditions. that compose it (Figure highland mountainous at depth Permeability on the younger upper4). layersTherefore, of the aquiferinis the highly variable due to the differentregion kinds of and fractures and rainfall snowmelt infiltrate features (lavaand tubes, pillow lava,can columnar basalt quickly. and individual lava flows) that compose it (Figure 4). Therefore, in the highland mountainous region and at depth rainfall and snowmelt can infiltrate quickly. Water sampling and analysis summary: Sixty-four water samples were taken from several purged groundwater wells near the INEEL screened in the upper part of the ESRiPA aquifer. All samples were filtered to 0.45 ȝm on site, preserved with Nitric Acid (HNO3) and stored in acid washed HDPE high isotopic ratio zone. The inverse is also true for the low isotopic ratio zones (Figure 5 B-D). Water-rock interactions affect groundwater elemental concentrations through different processes, including mineral dissolution, ion exchange, desorption and adsorption. Unlike Water sampling and analysis summary: elemental concentrations, isotopic Sixty-four water samples were taken from ratios are unaffected by several those purged groundwater wells near the INEEL processes, because the Uranium screened the upper parttransferred of the ESRiPAto the and inStrontium aquifer. All samples were filtered toidentical 0.45 μm to water is isotopically on site, preserved with Nitric Acid (HNO ) 3 the host rock. With this important and stored in acid washed HDPE bottles. fact in mind both isotopic ratios The samples were analyzed for uranium and (Uranium and Strontium) suggest a strontium isotopic composition as well as major channeling of radiogenic elemental chemistry, using different analytical groundwater from the north, methods such as Thermal Ionization Mass through (Uranium, zonesStrontium of isotopes), relatively Spectrometry unradiogenic and(anions). around ICPMS (cations), Ionmaterials chromatography the western and central zones that have low isotopic ratios. These Results of tracer studies: same preferential flow paths Figure 5 shows the result of the sample analysis appear to cause some sort of as a contour plot interpolated using the stagnation of method. watersThewith KRIGIN interpolation contourlow isotopes ratios (CLZ, WLZ) plots of the Uranium and Strontium isotopic (Figure 5D). ratios show water masses with high isotopic ratios emanating from Little Lost River and One key about Birch Creek thatfact penetrate waterthese massesaquifer of is that the host basalt rock lowsystems isotopic ratios originating elsewhere in the aquifer. distinct water massesaquifer continue are andThese sediments in the for characterized tens of kilometers away from their by low apparent isotopic sources. Also it is important to note the low and ratios while mountain ranges isotope ratio zones in the Southwest and their sediments (Paleozoic and Central regions, near the INEEL Rocks) site (Figure are Precambrian Clastic 5 A and E). The elemental concentrations characterized by high isotopic data also reflect some spatial similarities to ratios. Therefore we can use the the isotopic ratio data. For example high Mg isotopic ratios as an indicator of concentration and low Na and Si concentration water-rock contact time when slightly resembles the high isotopic ratio zone. totrue elemental Thecompared inverse is also for the lowchemistry. isotopic ratio zones (Figure 5 B-D). Figure 4. Surficial features show the characteristic type of openings in the Pliocene and younger basaltic-rock aquifers. A and B show, undulating surface of a basalt flow with collapsed and partly collapsed lava tubes. C and D shows pillow basalt raging from large boulders to tand grain sand that formed when lava entered water and cooled quickly. E shows side view of columnar basalt showing typical cooling joints. These openings provide easy access for water from recharge sources to enter the underlying aquifer. The results then suggest that the Water-rock interactions affect groundwater elemental concentrations through different processes, including mineral higher the isotopic value the dissolution, ion exchange, desorption and adsorption. Unlike elemental concentrations, isotopic ratios are unaffected shorter the residence time of by those processes, because the Uranium and Strontium transferred to the water is isotopically identical to the ground waters. Nevertheless, the host rock. With this important fact in mind both isotopic ratios (Uranium and Strontium) suggest a channeling of data suggest that preferential flow radiogenic groundwater from the north, through zones of relatively unradiogenic materials and around the western and central zones that have low isotopic ratios. These same preferential flow paths appear to cause some sort of stagnation of waters with low isotopes ratios (CLZ, WLZ) (Figure 5D). One key fact about these aquifer systems is that the host basalt rock and sediments in the aquifer are characterized by low isotopic ratios while mountain ranges and their sediments (Paleozoic and Precambrian Clastic Rocks) are characterized by high isotopic ratios. Therefore we can use the isotopic ratios as an indicator of water-rock contact time when compared to elemental chemistry. is not the only mechanisms acting in the aquifer. As the elemental chemistry suggests, mixing of ground waters also plays an important role. Other alternate interpretations of the data were explored but failed to adequately explain the observed phenomena. Figure 5. Contour plots of (A) Strontium Isotopic Ratios, (B) concentration of (B) Si, (C) Mg, (D) Na and (E) Uranium Isotopic Ratios. (E) also shows location of river channels. Arrows indicate direction of groundwater influxes. Findings and conclusions: The results then suggest that the higher the isotopic value the shorter the residence time of ground waters. Nevertheless, the data suggest that preferential flow is not the only mechanisms acting in the aquifer. As the elemental chemistry suggests, mixing ground also plays an importantflowpaths role. Other in alternate interpretations Sr and U isotope ratios wereofused towaters distinguish preferential aquifer systems of the data wereofexplored but failed topredictable adequately explain the observed phenomena. because their relatively reactions. Therefore in the ESRiPA, in the vicinity of the INEEL it was determined that the mixing of groundwaters from the mounts of Birchand Creek and Little Lost River with waters from the east and south seems to be an Findings conclusions: important process. More important was the fact that two preferential flow pathways Sr and U isotope ratios were used to distinguish preferential flowpaths in aquifer systems because of their relatively extending southeast from these two rivers dominate the entire flow system in this area. predictable reactions. Therefore in the ESRiPA, in the vicinity of the INEEL it was determined that the mixing of The findings of these studies illustrate the importance of understanding fractured flow to groundwaters from the mounts of Birch Creek and Little Lost River with waters from the east and south seems to the management of groundwater resources and contaminant transport. be an important process. More important was the fact that two preferential flow pathways extending southeast from these two rivers dominate the entire flow system in this area. The findings of these studies illustrate the importance of understanding fractured flow to the management of groundwater resources and contaminant transport. Take home message: Take home message: Using radiogenic isotope ratios together with elementalwith chemistry in the ESRiPA madeinpossible the identification Using radiogenic isotope ratios together elemental chemistry the ESRiPA made of ground waterthe sources, preferential of flow paths, together with a better understanding of paths, the chemical and physical possible identification ground water sources, preferential flow together with processes in a highly heterogeneous aquifer system. This case study shows that a similar approach could potentially be used in other heterogeneous aquifer systems. Credits: Case study was written by Carlos D. Soto, Department of Hydrology and Water Resources, University of Arizona, Tucson. Original work by Johnson et al. (2000) and Roback et al. (2001) Further reading: Friedman, L. (2006) Hydrology of Fractured Rocks. Accessed 11/21 2006. http://water.usgs.gov/nrp/proj.bib/hsieh.html Geller, D. (2006) Eastern Snake River Plain Aquifer. Accessed 11/21 2006. http://academic.emporia.edu/schulmem/hydro/ TERM%20PROJECTS/Geller/Eastern%20Snake%20River%20Plain%20Aquifer.html Johnson, T. M., Roback, R. C., McLing, T. L., Bullen, T. D., DePaolo, D. J., Doughty, C., Hunt, R. J., Smith, R. W., Cecil, L. D., Murrell, M. T. (2000) Groundwater “fast paths” in the Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers. Geology 28(10), 871-874. Roback Robert C, Johnson Thomas M, McLing Travis L, Murrell Michael T, Luo Shangde, Ku, T. L. (2001) Uranium isotopic evidence for groundwater chemical evolution and flow patterns in the eastern Snake River Plain Aquifer, Idaho. Geological Society of America Bulletin 113(9), 1133-1141. Schramke Janet A, Murphy Ellyn M, Wood, B. D. (1996) The use of geochemical mass-balance and mixing models to determine groundwater sources. Appl. Geochem. 11(4), 523-539. Whitehead, R. L. GROUND WATER ATLAS of the UNITED STATES: Idaho, Oregon, Washington. Accessed 11/21 2006. http://capp.water. usgs.gov/gwa/ch_h/H-text8.html Wood, W. W. and Low, W. H. (1987) Solute Geochemistry of the Snake River Plain Regional Aquifer System, Idaho and Eastern Oregon. Available from Books and Open File Report Section CO 80225. USGS Open-File Report 86-247, 132 ref.