Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

By P.B. McMahon,¹ J.K. Böhlke,¹ and T.M. Lehman²

¹U.S. Geological Survey
²Texas Tech University, Lubbock, Texas

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Scientific Investigations Report 2004-5053, 53 p., 13 figs.

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The citation for this report, in USGS format, is as follows:
McMahon, P.B., Böhlke, J.K., and Lehman, T.M., 2004, Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002: U.S. Geological Survey Scientific Investigations Report 2004-5053, 53 p.

Abstract

The southern High Plains aquifer is the primary source of water used for domestic, industrial, and irrigation purposes in parts of New Mexico and Texas. Despite the aquifer's importance to the overall economy of the southern High Plains, fundamental ground-water characteristics, such as vertical gradients in water chemistry and age, remain poorly defined. As part of the U.S. Geological Survey's National Water-Quality Assessment Program, water samples from nested, short-screen monitoring wells installed in the southern High Plains aquifer at two locations (Castro and Hale Counties, Texas) were analyzed for field parameters, major ions, nutrients, trace elements, dissolved organic carbon, pesticides, stable and radioactive isotopes, and dissolved gases to evaluate vertical gradients in water chemistry and age in the aquifer. Tritium measurements indicate that recent (post-1953) recharge was present near the water table and that deeper water was recharged before 1953. Concentrations of dissolved oxygen were largest (2.6 to 5.6 milligrams per liter) at the water table and decreased with depth below the water table. The smallest concentrations were less than 0.5 milligram per liter. The largest major-ion concentrations generally were detected at the water table because of the effects of overlying agricultural activities, as indicated by postbomb tritium concentrations and elevated nitrate and pesticide concentrations at the water table. Below the zone of agricultural influence, major-ion concentrations exhibited small increases with depth and distance along flow paths because of rock/water interactions and mixing with water from the underlying aquifer in rocks of Cretaceous age. The concentration increases primarily were accounted for by dissolved sodium, bicarbonate, chloride, and sulfate.

Nitrite plus nitrate concentrations at the water table were 2.0 to 6.1 milligrams per liter as nitrogen, and concentrations substantially decreased with depth in the aquifer to a maximum concentration of 0.55 milligram per liter as nitrogen. Dissolved-gas and nitrogen-isotope data from the deep wells in Castro County indicate that denitrification occurred in the aquifer, removing 74 to more than 97 percent of the nitrate originally present in recharge. There was no evidence of denitrification in the deep part of the aquifer in Hale County. After correcting for denitrification effects, the background concentration of nitrate in water recharged before 1953 ranged from 0.4 to 3.2 milligrams per liter as nitrogen, with an average of 1.6 milligrams per liter as nitrogen. The δ¹⁵N composition of background nitrate at the time of recharge was estimated to range from 9.6 to 12.3 per mil.

Mass-balance models indicate that the decreases in dissolved oxygen and nitrate concentrations and small increases in major-ion concentrations along flow paths can be accounted for by small amounts of silicate-mineral and calcite dissolution; SiO₂, goethite, and clay-mineral precipitation; organic-carbon and pyrite oxidation; denitrification; and cation exchange. Mass-balance models for some wells also required mixing with water from the underlying aquifer in rocks of Cretaceous age to achieve mole and isotope balances. Carbon mass transfers identified in the models were used to adjust radiocarbon ages of water samples recharged before 1953. Adjusted radiocarbon ages ranged from less than 1,000 to 9,000 carbon-14 years before present. Radiocarbon ages were more sensitive to uncertainties in the carbon-14 content of recharge than uncertainties in carbon mass transfers, leading to 1-sigma uncertainties of about ±2,000 years in the adjusted ages. Despite these relatively large uncertainties in adjusted radiocarbon ages, it appears that deep water in the aquifer was considerably older (at least 1,000 years) than water near the water table.

There was essentially no change in ground-water age with depth in deeper parts of the aquifer, indicating that water in that part of the aquifer was vertically well mixed. Both sites are located in areas of intensive long-term irrigation; therefore, local irrigation pumping is the most likely explanation for vertical mixing. The absence of ground-water age gradients in the deep aquifer is an indication that pumping is likely to accelerate the downward movement of anthropogenic compounds like nitrate and pesticides from the water table to deeper parts of the aquifer. Denitrification rates for deeper parts of the aquifer estimated on the basis of nonatmospheric dissolved nitrogen gas concentrations and radiocarbon ages were slow, averaging about 3.5x10^–4 milligram per liter per year as nitrogen. Considering these slow denitrification rates, this process may not attenuate nitrate that is transported deeper into the aquifer by processes like pumping.

Contents

Foreword

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Study Area

Geohydrologic Setting

Land and Water Use

Study Methods

Vertical Changes in Lithology

Vertical Hydraulic Gradients

Vertical Gradients in Water Chemistry

Tritium

Dissolved Oxygen and Organic Carbon

Major Ions and Trace Elements

Nitrate

Pesticides

Vertical Gradients in Ground-Water Age

Mass-Balance Models

Radiocarbon Ages

Summary and Conclusions

References Cited

Appendix 1—Water-Quality Data from Monitoring Wells Screened in the Southern High Plains Aquifer and Dockum Group

Appendix 2קCalculated Phase Mass Transfers and Isotope Balances for Selected Pairs of Initial and Final Waters

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