Water resources and hydrochemistry of the Alluvial and Sparta aquifers of the Grand Prairie region
Chad D. Cooper, Ralph K. Davis, Kenneth F. SteeleUniversity of Arkansas, Department of GeosciencesArkansas Water Resources Center
Multiple ground-water uses Ground-water level declines Critical Ground-Water Area as of 1998 Salinization may be related to ground-
water level decline Water-quality variations resulting from
lower water levels can impact all uses
The Problem
Objectives Delineate spatial distribution and
magnitude of varying water quality conditions as related to areas of ground-water level decline
Determine areas of ground-water salinization (high in chloride, SpC, TDS)
Examine varying water-quality conditions spatially over the study area
Provide baseline water-quality data to water users of the region
Physiography of Study Area
Approximately 8,900 square kilometers
Little to no reliefDominated by
agricultural production
1999 Land Use/Land Cover(summer)
Water level, Alluvial aquifer, spring 1998
Declines in Alluvial:
1915-1933 – up to 60 feet, Arkansas Co.
1938-1953 – from 10-20 feet
1955-1961 – from 3-10 feet
1969-1996 - 32 feet, Lonoke Co.
Wells completed in the Alluvial aquiferAbout 7,000 wells!
Water level, Sparta aquifer, spring 1998 Declines in Sparta:
1949 – 50 feet near Pine Bluff
1958-1965 – 160 feet at Pine Bluff
1969-1999 – 70 feet+ in Arkansas Co.
1988-1999 – almost 20 feet in Lonoke Co.
Wells completed in the Sparta aquifer
About 325 wells (and rising)!
Sample Sites
July/Aug 2000
78 Alluvial 39 Sparta 8 QA/QC Analyses
at WQL of AWRC
Ground-water quality, Alluvial aquifer
Good to acceptableCalcium-
magnesium-bicarbonate type
No MCL’s exceededSMCL’s exceeded:
Mn (0.05 mg/L)Fe (0.3 mg/L)
Chloride: 3-172 mg/L
Ground-water quality, Sparta aquifer
BetterRange from Ca-Na-
HCO3 type to Na-HCO3 type
No MCL’s exceededSMCL’s exceeded:
Mn (0.05 mg/L)Fe (0.3 mg/L)
Chloride: 2-56 mg/L
Salinization related to declines?
Cl, Alluvial Aquifer Cl, Sparta Aquifer
Salinization related to declines?
TDS, Alluvial Aquifer TDS, Sparta Aquifer
Spatial trends, Sparta aquifer
GW
FLOW
Lonoke
Jefferson
Arkansas
Prairie
Spatial trends, Sparta aquifer
Ionic Strength Calcium Saturation
Spatial trends, Sparta aquifer
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100
Distance from northwest end (km)
Cal
ciu
m a
nd
Sod
ium
(m
g/L)
0
10
20
30
40
50
60
70
80
90
Sod
ium
/Cal
ciu
m (
meq
/L)
Ca
Na
Na/Ca
LSA76
LSA46
PSA67
LSA92725
ASA74
PSA61
PSA76
ASA23
ASA72
ASA43 ASA7
ASA15
ASA85
AMI5
ASA67
Transect Location
Results
GW
FLOW
Spatial trends, Alluvial aquifer
Conductivity vs. Well Depth
Spatial trends, Alluvial aquifer
Ionic StrengthCalcium Saturation
37 of 78 samples saturated
Spatial trends, Alluvial aquifer
pH vs. Ca Saturation
pH~6.85
Spatial trends, Alluvial aquifer
pH vs. Iron
pH~6.85
Results and Conclusions
Salinization not widespreadWater quality for both aquifers:
Alluvial: Acceptable, with high Cl and Fe Sparta: Better, lower Cl and Fe
Varying hydrochemistry not result of ground-water level declinesVarying hydrochemistry is instead result of geochemical evolution along flowpath
Results and ConclusionsDominant processes:
Alluvial: Ca saturation, precipitation of calcium- and iron-rich cementsSparta: Ca saturation, precipitation of calcium-rich cement; cation exchange
Future work:Combine and compare with other datasets from the regionStatistical analysis of spatial dataExamine relationships with riversIron in the Alluvial aquifer?
AcknowledgementsArkansas Water Resources Center
(AWRC)Water Quality Lab, AWRCArkansas Soil and Water Conservation
CommissionUnited States Geological Survey, Little
RockArkansas Department of
Environmental QualityCenter for Advanced Spatial
Technologies, University of Arkansas
Questions?
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