CHARACTERIZATION OF THE GROUNDWATER FLOW SYSTEM OF SOUTHERN BALDWIN COUNTY, ALABAMA USING MULTIISOTO

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CHARACTERIZATION OF THE GROUNDWATER FLOW SYSTEM
OF SOUTHERN BALDWIN COUNTY, ALABAMA USING
MULTI-ISOTOPE DATA
Alabama Water Resources Conference September,
2009 DORINA MURGULET
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OUTLINE

• Introduction
• Objectives
• Study area and background information
• Methodology
• Results
• Conclusions

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Introduction

• Southern Baldwin County Alabama has experienced
  continuously increasing growth and development
  over the last decade. Consequently, groundwater
  in the deeper aquifers in this area has been
  increasingly targeted for the regions growing
  municipal and industrial water needs.
• Expanding use of groundwater resources requires
  better knowledge about groundwater recharge and
  flow of the regional groundwater system.

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Objectives

• to develop a better understanding of the regional
  flow system, with respect to recharge mechanisms
  and groundwater residence times of the aquifer
  system in southern Baldwin County
• to characterize the local and regional flow
  system in the aquifers beneath southern Baldwin
  County

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Location of the study area and surveyed
groundwater wells
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Generalized Hydrogeologic Cross Section of
Southern Baldwin County, Alabama
Aquifer zone A2 (the Gulf Shores aquifer)
Aquifer zone A3 (the Deep Miocene aquifer)
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Methodology

• To meet the objectives of the study the
• following analysis were conducted
• dD and d18O of groundwater
• d15N and d18O of groundwater NO3-
• d13C, percent Modern Carbon (pMC) and groundwater
  ages
• Recharge areas delineation

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Background Information

• Isotope measurements complement geochemical and
  hydrogeologic observations by providing
  identification of source waters and recharge
  rates.
• However, when applying these methods to regional
  and complex flow systems, data interpretation
  becomes very difficult.

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• Results

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• the groundwater d13C data ranges between -13.5
  and -19.2 indicative of a plant material
  signature (e.g., Hatch-Slack cycle).
• the groundwater in this area originates from
  locally infiltrated precipitation through soils
  extensively cultivated with plants such as corn,
  grass, and millet, which are widespread in
  temperate zones (Tamers, 1975).

13?VPDB ()
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dD4.1d18O-3.03
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a)
b)
Weighted annual d18O (a) and dD (b) for North
America (from IAEA/WMO (2006)).
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• Based on d18O and dD isotope value distribution
  with depth, no preferential isotope signature was
  identified for different aquifers.
• No correlations were observed between latitude
  and d18O and dD values or between aquifer depth
  and d18O and dD values.

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Mixing and Denitrification
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Mixing and Denitrification
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Mixing and Denitrification
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Mixing and Denitrification
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Background Information

• Aquifers with short
• residence times and low
• DOC concentrations are
• mainly oxic.

• Aquifers with long residence times and high DOC
  concentrations are mainly anoxic
  denitrification.

Krest et al.,2000
Schematic representation of the organic matter
decomposition pathways in two end-member systems
with low DOC input a shallow aquifer with a
short residence time and a deep aquifer with a
long residence time.
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• Groundwater residence times in aquifer A2
  ranged up to 3,030 radiocarbon years, while those
  in aquifer A3 ranged between 305 to 7,600
  radiocarbon years.

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• Increasing A14C values in groundwater were
  observed down-gradient and with depth, indicating
  the presence of downward leakage from the upper
  aquifer A2 to the lower aquifer A2 and deeper to
  aquifer A3.
• Replenishment rates are estimated to range from
  0.04 to 0.37 m/yr locally

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• The relationship between d18O and A14C values
  suggests that locally derived groundwater with
  high 14C content mixes continually along flow
  paths with lower 14C ambient groundwater.

Aquifer A3
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Recharge areas designation and classification
LULC
Soil drainage
Runoff
Soil conductivity
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Conclusions

• The d13C and d18O of groundwater data indicates
  that water in the aquifer system of the study
  area is most likely to have originated from
  precipitation and soil infiltration through
  relatively localized recharge.
• the d15N and d18O of NO3- indicates that the
  nitrate behavior can be explained by mixing of
  different end-member waters (different isotopic
  signatures and NO3- concentrations).

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Conclusions

• The narrow range of dD and d18O values reflects
  the local meteoric origin of groundwater that
  precipitation infiltrates rapidly to depth and
  does not undergo significant evaporation at the
  ground surface - any evaporation that may take
  place likely occurs before the precipitation
  water reaches the land.

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Conclusions

• Because recharge occurs in the absence of
  evaporation, it can be implied that surface water
  has negligible contribution to groundwater
  replenishment. This further suggests the high
  potential of contaminant input from groundwater
  to surface water
• Given the regional distribution of the isotope
  data the results of this study suggest the
  presence of multiple recharge areas within the
  region of southern Baldwin County and therefore
  the presence of considerable groundwater mixing
  along the flow paths.
• These processes may explain the presence of
  unexpected young groundwaters at greater depths
  along flow paths.

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Conclusions

• Overall, the radiocarbon data indicate that
  waters in deeper aquifers A2 and A3 have been
  recharged beginning in the early Holocene.
  However, middle and late Holocene recharge waters
  are pervasive in the aquifer system of southern
  Baldwin County. This suggests that water is not
  stagnant in these aquifers and that it is either
  discharging into the Gulf of Mexico or is being
  pumped for irrigation and domestic uses.
• The isotope data and recharge areas distribution
  indicate the increased vulnerability to
  contamination of these highly developed aquifers.

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Acknowledgments

• Founding
• NOSAMS
• Geological Survey of Alabama
• AWARRI
• ADCNR
• People of Baldwin County
• Dr. Geoffrey Tick
• Dr. Aharon and Joe Lambert
• Betsy Graham
• ..

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Thank you
dmurgulet_at_bama.ua.edu