Forensic Hydrology
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Transcript of Forensic Hydrology
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Mark Williams, CU-Boulder
Forensic Hydrology
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What is “Forensic Hydrology”
Geoscientists are really “Geodectives” forensic geochemistry and forensic geology
were terms to describe the use of geochemical or geological techniques to identify potential sources of contamination.
approach is predicated on forensic earth sciences.
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Tool Chest of the Forensic Hydrologist
Flowpath examinationGeochemical analysisWater isotopesFingerprinting techniques
be used to trace specific contaminants of concern (CoCs).
Element/element ratios, isotopes, leachate waters Don’t over-interpret the use of “fingerprinting”!
Choose the right mix
Hurst, SWH 2008
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Case Study: Mojave powerplant fly ash
Hurst, SWH 2008
Were heavy metals present in flyash generated by coal combustion being transported to local surface waters and water in embayments along the Colorado River?
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Case Study: Consumptive use of Colorado River water
DISPUTE: Groundwater wells in AZ and CA pumping Colorado River water? Several thousand wells
SOLUTION: Reclamation proposed an “accounting surface” method to address wells outside the flood plain.
The method relies on a hydraulic criterion: wells that have a static water-level elevation equal to or below the published accounting surface are presumed to yield water that will be replaced by water from the river.
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Case Study: Consumptive use of Colorado River water
CRITICISM: the method does not provide direct evidence that a well yields mainstream water.
SOLUTION: A method that could distinguish between withdrawals of mainstream or locally recharged tributary water would be a welcome advance.
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Guay and Eastoe, SWH 08
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QuickTime™ and a decompressor
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Map of Colorado River watershed showing theLower Colorado River Basin (shaded) and Topock Marsh study area
Guay and Eastoe, SWH 08
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3 distinct source waters
Locally recharged precipitation“Older (pre-1950’s)” groundwater
Perhaps even pre-dam waters
Third, there is “recent” (post-1950) Colorado River water, which includes Topock Marsh samples.
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Forensic Insights
Without isotopic data, the accounting surface falls short because it can only demonstrate the physical potential for water movement from the river toward a well.
Conventional geochemical data are useful but rarely provide a direct indication of a water’s source.
Isotopes, on the other hand, can and have resolved water resource disputes in many situations.
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Combine forensic approach with awatershed approach
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WHY WATERSHED APPROACH?
Explicitly incorporates spatial dimensionsTreat streamwater as a mixture of source
waters and/or flowpathsApply to any water body
-wells, lakes, wetlands, storm drainage, etc
Develop inverse models that allow us to “unmix” samples and identify source waters and flowpaths
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WATER QUALITY IN STREAMS ANDRIVERS IS THE END PRODUCT OF ALL
PROCESSES IN THE BASIN
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Kim Raby collects water quality samples outside of Silverton, Colorado
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MIXTURE APPROACH
APPLY THE SAME CONCEPTS TO OTHER HYDROLOGIC FEATURES IRRIGATION SYSTEMS WELLS WETLANDS SETTLING PONDS SEPTIC SYSTEMS
SOURCES OF WATER TO SYSTEMSFATE OF WATER FROM THESE SYSTEMS
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Watershed management in fractured-rock settings
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SOURCE WATERS
Initial source of water that contributes to a mixture: stream, well, settling pond, etc
Empirically defined“OLD” is water stored prior to the
precipitation event“NEW” is water from current precip event
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SOURCE WATERS
Two main types: PRECIPITATION GROUNDWATER (FRACTURE FLOW)
Can be septic system release, landfill plume, acid mine drainage, irrigation water, etc
Main requirement is that the different source waters have UNIQUE GEOCHEMICAL/ISOTOPIC SIGNALS
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FLOWPATHS
Flowpaths are the routes that water takes from the source area to the mixture
The hydrogeologic setting of the flowpath alters the geochemical/isotopic content of the source water
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FLOWPATH TYPES
Hortonian Overland FlowSaturation Excess FlowReturn FlowGroundwaterPiston PumpingTranslatory Flow
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Challenge to forensic hydrology
Each of these source waters has a unique geochemical and isotopic fingerprint
Each of the flowpaths contributes a unique signiture to the “mixture”
Challenge: unmix “FINGERPRINTS” measured in stream flow to quantify source waters and flowpaths
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Hortonian Overland Flow
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Infiltration greater than thought
Measure rate of fall in inner ring
Infilration
http://www.alwi.com/wastewater.php
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Hydrology 101
Old reliance on Hortonian overland flow is not valid
Much higher rates of infiltration than previously thought
“Groundwater” not a uniform body because of preferential flowpaths
Isotopes combined with watershed approach can help
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Summary
Forensic approach. Toolchest includes Flowpath analysis Water isotopes Geochemical tracers “Environmental fingerprints”
Combine with watershed approachIdentify sources of contaminants of concern
by “unmixing” water samples