Kansas Geological Survey, Open-file Report 88-21

Dating of Ground Water with Isotopes--A Preliminary Report

by
Sabine E. Bock and Howard G. O'Connor

KGS Open File Report 88-21
March 1988

Introduction

The purpose of this project is twofold:

1. identify potential recharge areas;
2. gain information about the recharge mechanisms, i.e.
   1. how important are, for example, lineaments, intermittent streams and playa-like structures for contributing water to the Ogallala aquifer?,
   2. at what rate is the aquifer being recharged?

Studies using mathematical models to determine recharge rates and areas, flow, and general aquifer conditions of the Ogallala have been conducted (Reddell, 1970, Claborn, Austin, and Wells, 1970, Rayner, 1970), but no comparison was possible with actual field data. Recharge is a very sensitive parameter as far as models are concerned and a factor of "insecurity" as to how accurate and reliable those models are always remains.

It was decided to test the Ogallala aquifer for environmental (tritium, ³H) and stable (oxygen-18, ¹⁸O; deuterium, ²H) isotopes, freon, atrazine and nitrates in order to be able to distinguish between older and modern waters and thus gain information about potential recharge areas and the rate of flow. In doing so a comparison of field data with existing models would be possible and the reliability of the models would be established or questioned.

Results of this project will help to improve water management in the investigated area, the management district, the water users, and those regulatory agencies concerned with sanitary- and hazardous-waste disposal will benefit.

Northwest Kansas Groundwater Management District 4 was chosen as a study area (Fig.1).

Figure 1--Study area

The project was divided into two phases. Phase 1 was concerned with establishing the most suitable analytical methods to date the Ogallala ground water, and phase 2 will expand the sampling net in both the saturated and unsaturated parts of the Ogallala using the isotopes determined to be the most suitable.

For phase 1, nine wells were selected in early October 1987 in cooperation with the management district, using mainly proximity to lineaments as a selecting criterion (Fig. 2). Seven of these wells were already existing wells, which we had information about and permission to use for sample collection. Because of lack of time and funds, only two monitoring wells were installed in late October 1987 by the Kansas Geological Survey. Table 1 gives the designated well names (also used for labeling water samples taken from the respective wells), legal locations, and counties for all nine wells. Table 2 gives total depth, screened depth, and use of the wells.

Figure 2--Lineaments and well locations (from McCauley, 1988)

Table 1--Well Designations and Locations of Ogallala Wells.

Table 2--Total Depth, Screened Depth, and Use of Wells.

From November 10 through November 13, 1987, we collected water samples from the wells for the isotopes ¹⁸O, ²H and ³H, and freon, atrazine, nitrate and general water chemistry. Where possible the samples were taken using a Bennett sample pump, provided by GWMD #4. In addition one precipitation sample was collected by the Kansas State University Experiment Station at Colby on November 8.

The complete text of this report is available as an Adobe Acrobat PDF file.

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Kansas Geological Survey, Geohydrology
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