environmental setting, water quality, and ecological indicators of

Major Inorganic Ions and Trace Elements
Dissolved concentrations of 7 major inorganic ions
were determined for ground- and surface-water samples,
and dissolved concentrations of 16 trace elements were
determined for samples from all wells and one surfacewater
site, Mermentau. Concentrations of major inorganic
ions and trace elements were compared with
MCL’s and SMCL’s (U.S. Environmental Protection
Agency, 2000a). MCL’s were established to protect public
health by limiting the levels of contaminants in drinking
water; SMCL’s were established to limit cosmetic
(for example, skin or tooth discoloration) or aesthetic
affects (for example, taste, odor, or color) in drinking
water. No MCL’s were exceeded in either ground- or surface-water
samples; however, one or more SMCL’s were
exceeded in samples from 20 wells and all the surfacewater
sites (fig. 8). Concentrations of antimony, arsenic,
barium, beryllium, cadmium, chromium, copper, lead,
selenium, and thallium generally were below detection
limits, but when detected did not exceed MCL’s.
Median concentrations of calcium and magnesium
in ground water were more than three times the median
concentrations in surface water (fig. 8). The median concentrations
for calcium and magnesium in ground water
were 66.0 mg/L and 25.2 mg/L; maximum concentrations
occurred at well 35 (129 mg/L) and well 8
(61.1 mg/L). The median concentrations for calcium and
magnesium in surface water were 15.9 mg/L and
7.02 mg/L; maximum concentrations occurred at Theriot
(69.3 mg/L) and Lacassine (38.5 mg/L), sites with high
agricultural intensity (table 7).
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Median potassium concentrations were about three
times higher in surface water than in ground water
(fig. 8). The maximum and median potassium concentrations
in ground water were 3.61 mg/L and 1.52 mg/L
and in surface water were 22.8 mg/L and 4.31 mg/L. The
maximum concentration detected in ground water was at
well 10 and in surface water was at Theriot (table 7).
Potassium concentrations in surface water peaked twice
annually (fig. 10), in mid-spring and late fall; peaks
probably were caused by runoff from fertilizer applications
and by re-solution of detrital plant material (Hem,
1992).
Median concentrations of sodium and chloride
(fig. 8) were higher in ground water than in surface
water, but the highest concentrations of both constituents
occurred in surface water (fig. 8). The median sodium
concentration in ground water was 64.3 mg/L and the
median chloride concentration was 101 mg/L; the highest
concentrations were 275 mg/L at well 7, and
328 mg/L at well 22 (table 7). Chloride concentrations
in ground water exceeded the SMCL (250 mg/L) (U.S.
Environmental Protection Agency, 2000a) in four wells:
well 35 (264 mg/L), well 25 (281 mg/L), well 7
(293 mg/L), and well 22 (328 mg/L). The median chloride
and sodium concentrations in surface water were
33.4 mg/L and 33.0 mg/L. The maximum concentrations
of sodium (327 mg/L) and chloride (586 mg/L) in
surface water occurred at Lacassine (table 7) in March
2000 and probably resulted from saltwater intrusion
induced by flow reversal (bidirectional) during the
drought. The chloride concentration exceeded the
SMCL (U.S. Environmental Protection Agency, 2000a).
Table 7. Maximum concentrations and occurrence of selected major inorganic ions in ground water and surface water
in the Mermentau River Basin, southwestern Louisiana, 1998-2001
[mg/L, milligrams per liter; µg/L, micrograms per liter]
Major inorganic ion
Ground water Surface water
Concentration Well number Concentration Site name
Calcium, in mg/L 129 35 69.3 Theriot
Magnesium, in mg/L 61.1 8 38.5 Lacassine
Potassium, in mg/L 3.61 10 22.8 Theriot
Sodium, in mg/L 275 7 327 Lacassine
Chloride, in mg/L 328 22 586 Lacassine
Alkalinity as calcium
carbonate, in mg/L
546 10 380 Theriot
Sulfate, in mg/L 272 10 79.2 Lacassine
Iron, in µg/L 2,190 49 1,430 Chene
Manganese, in µg/L 739 9 5,270 Boggy