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Areal, Vertical, and Temporal Differences in Ground Water Chemistry: I. Inorganic Constituents¹

ABSTRACT

Definite areal, vertical, and temporal differences were observed in the ground water chemistry of a 62 km² area in Nebraska's Platte Valley. A network of 53 wells was monitored 10 times between July 1975 and July 1976 for dissolved solids, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^–, SO₄^2–, Cl^–, F^–, NO₃-N, PO₄-P, SiO₂, pH, and temperature.

Areal differences were most pronounced in the shallow ground water and reflected upgradient land use and soil type. Average total dissolved solids (TDS) and nitrate-nitrogen (NO₃-N) concentrations ranged from 557 to 2,210 mg/liter and from 0.1 to 33 mg/liter, respectively, with the lowest concentrations occurring downgradient from near-pristine areas and the highest concentrations occurring downgradient from cultivated and irrigated fields.

In ground water downgradient from cultivated and irrigated fields, most of the chemical parameters had pronounced decreases in concentration with depth. Stratification was more pronounced in the bottomland than in the terrace and in the latter reflects mixing of the ground water caused by intensive pumping of irrigation water during the last four decades. Magnesium and potassium had marked decreases with depth regardless of upgradient land use.

Marked cyclic fluctuations in NO₃-N and SO₄^2– concentrations generally were observed in the bottomland ground water. Sulfate concentrations, which were significantly correlated with TDS, Ca²⁺, Mg²⁺ Na⁺, and Cl^– (r +0.89), peaked in late winter and early spring.

Ground water in secondary aquifers confined within or beneath a layer of clayey silt had an average TDS concentration of 304 mg/liter and almost negligible amounts ( 0.1 mg/liter) of NO₃-N. Temporal fluctuations in chemical constituents from these aquifers were the smallest observed within the study area.

Key Words: ground water monitoring • land use • river seepage • leaching

¹ Contribution from the Conserv. and Survey Div., Inst. of Agric. and Nat. Resour., Univ. of Nebraska, Lincoln, NE 68588. This research was funded through a grant from the Central Platte Nat. Resour. District.

² Associate Professor and Assistant Professor, respectively.

Received for publication September 24, 1979.

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