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Ordering Information:
ORDER NUMBER: 90941F
DATE AVAILABLE: Winter 2002/2003
Printed Report |
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PRINCIPAL INVESTIGATOR:
Mikel S. Halliwell
OBJECTIVES:
The purpose of the project was to demonstrate the value of age-dating and isotopic
tracers in characterizing the flow dynamics and water quality changes in a complex
groundwater domain that includes high capacity municipal pumping wells, a geologic
fault, and artificial recharge facilities with deep lake-like recharge ponds.
Characterizing water quality changes during recharge and transport in groundwater
was also an objective of this investigation.
BACKGROUND:
Much has been written about application of isotopic methods for characterizing
groundwater and recharge. However, there has been a lack of case studies documenting
the introduction of isotopic tracers in deep recharge ponds and their subsequent
migration in groundwater. Use of artificial tracers in conjunction with other
isotopic methods could help fulfill a desire among many well operators and basin
managers for a reliable means of identifying preferential flow paths, amount
of dilution, and minimum times of travel from surface water sources to wells.
The desire for such characterization stems in part from increasing regulatory
and public concerns regarding pathogens.
HIGHLIGHTS:
Below (west of) the Hayward Fault (BHF), water ages correlated well with aquifer
layer sequence. BHF tracers did not reach the BHF wellfield within the time
frame of the experiment. Above (east of) the fault, (AHF) tracers reached the
targeted wellfield in only 60 days, indicating substantial heterogeneity and
a fast travel time along preferential pathways compared to the average travel
time of 2+ years indicated by age-dating and more classical estimating methods.
A reconnaissance of water quality, conducted concurrently with the tracer studies,
suggested certain water quality improvements occurring in either the pond sediment
or the near-pond aquifer media.
APPROACH:
The first phase of work involved sampling groundwater for natural isotopes.
Groundwater ages were determined by the tritium-helium method. Oxygen-18 and
Deuterium were also measured to differentiate between local versus imported
sources of recharge. Carbon isotopes were measured to provide additional insight
on water age and to infer geochemical processes affecting dissolved inorganic
carbon.
Tracer experiments followed the initial study. Noble gas tracers were added
to surface water in the recharge facilities. Use of multiple tracers enabled
tracking recharge contributions from individual ponds. Tracer concentrations
in surface water were recorded during dosing and for three weeks thereafter.
The progress of tracer migration in groundwater was tracked for approximately
one year. Samples for characterization of water chemistry were also collected
during the tracer-monitoring period.
RESULTS/FINDINGS:
Variations in groundwater age depended on location and depth. A survey of natural
isotopes indicated mixing of young and older water in wells, increasing age
with depth of aquifer layer, and possible dissolution of carbonate minerals.
AHF tracer experiments, along with other analysis, suggested that tracers probably
percolated preferentially at shallow depths in the pond near the shoreline.
Much of the tracer remained in deep pond water over time, increasing residence
time in down-gradient wells. The tracer studies provided evidence of preferential
pathways and heterogeneity in the AHF aquifer and a fast minimum travel time
to the AHF wellfield.
The tracer added to BHF ponds was detected in just two monitoring wells, but
not at the BHF wellfield over the 10-month period. An AHF tracer from a small
pond spanning the fault did, however, appear at the BHF wellfield. BHF pond
water flows mainly to distal portions of the groundwater basin, or reaches the
wellfield over a slow, circuitous route.
Documentation of water chemistry changes during recharge and subsequent migration
in the aquifer was not achieved with the desired level of precision, but nevertheless
provided evidence of improvements to water quality (reductions in TOC, MTBE,
nitrate, and coliphage).
IMPACTS:
Well operators, groundwater basin managers, regulators, consultants, and researchers
may gain insight on the benefits and limitations of the isotopic-related methodologies
followed in this study with respect to the following:
• Assessing vulnerability/resiliency of municipal wells in proximity to
surface waters or potential sources of contamination
• Evaluating heterogeneity/preferential pathways within flow-domains of
limited aerial extent
• Investigating pond hydrodynamics
• Evaluating seepage from surface waters
This study may serve as useful case study or basis for additional research on
the mechanics of recharge from deep recharge ponds and water quality changes
experienced during recharge.
RESEARCH PARTNER:
Alameda County Water District