Mark Williams, CU-Boulder Forensic Hydrology

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.

Tool Chest of the Forensic Hydrologist Flowpath examination Geochemical analysis Water isotopes Fingerprinting 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

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?

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.

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.

Guay and Eastoe, SWH 08

Map of Colorado River watershed showing the Lower Colorado River Basin (shaded) and Topock Marsh study area Guay and Eastoe, SWH 08

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.

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.

Combine forensic approach with a watershed approach

WHY WATERSHED APPROACH? Explicitly incorporates spatial dimensions Treat streamwater as a mixture of source waters and/or flowpaths Apply 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

WATER QUALITY IN STREAMS AND RIVERS IS THE END PRODUCT OF ALL PROCESSES IN THE BASIN

Kim Raby collects water quality samples outside of Silverton, Colorado

MIXTURE APPROACH APPLY THE SAME CONCEPTS TO OTHER HYDROLOGIC FEATURES  IRRIGATION SYSTEMS  WELLS  WETLANDS  SETTLING PONDS  SEPTIC SYSTEMS SOURCES OF WATER TO SYSTEMS FATE OF WATER FROM THESE SYSTEMS

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

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

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

FLOWPATH TYPES Hortonian Overland Flow Saturation Excess Flow Return Flow Groundwater Piston Pumping Translatory Flow

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

Hortonian Overland Flow

Infiltration greater than thought Measure rate of fall in inner ring Infilration

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

Summary Forensic approach. Toolchest includes  Flowpath analysis  Water isotopes  Geochemical tracers  “Environmental fingerprints” Combine with watershed approach Identify sources of contaminants of concern by “unmixing” water samples