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 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

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

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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

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 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

WATER QUALITY IN STREAMS ANDRIVERS IS THE END PRODUCT OF ALL

PROCESSES IN THE BASIN

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

Watershed management in fractured-rock settings

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 FlowSaturation Excess FlowReturn FlowGroundwaterPiston PumpingTranslatory 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

http://www.alwi.com/wastewater.php

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 approachIdentify sources of contaminants of concern

by “unmixing” water samples