Environmental Controls on Ground water Chemistry in New Mexico, The Effect of Phreatophytes

The relationship between phreatophyte-induced evapotranspiration, water level fluctuations, and changes in groundwater quality were investigated with arrays of nested piezometers installed at two sites of the Rio Grande flood plain. Only data from one site (Hope Farms), spanning 490 days, were found suitable for analysis and interpretation.

Consumptive use was computed from continuous water level records. The best correlation was found between consumptive use and averaged maximum daily temperature, while a lack of direct recharge from precipitation at the site was indicated.

The absence of a dominant salinity stratification in either space or time was the salient feature of the specific-conductance data. A strong seasonal cyclic variation was observed in the difference between horizontally averaged conductivities from piezometers set at 10 ft and 20 ft below the land surface. This cyclic variation showed a strong inverse correlation with water table fluctuations caused by evapotranspiration. This is in agreement with a salinity mechanism of temporary “deposition” of salts in and above the capillary fringe during the growing season, ascribed to the transpiring phreatophytes, and “dissolution” of these salts in the fall and early winter as the phreatic surface rises. The averaged horizontal water table gradient also showed a high inverse correlation with the specific conductance difference.

The chemical characteristics of the groundwater in this area appear to be determined by mixing of waters of different chemical composition. In general, the groundwater of this area has a calcium sulfate character.

It is postulated that evapotranspiration causes fluctuations of the vertical hydraulic gradient which are responsible for the mixing and a weak diurnal cycle of specific groundwater conductance.

Project No. A-030-NMEX