As part of development of a regional source water protection plan, in 2015–2016, the New Mexico Bureau of Geology and Mineral Resources performed a technical review of existing hydrogeology studies in Curry and Roosevelt counties in east-central New Mexico. Additionally, groundwater quality was tested in several wells, and groundwater levels were examined to provide up-to-date information on the availability of groundwater in the region. This report describes the results of the hydrogeologic review and findings from the groundwater study.
In Curry and Roosevelt counties, irrigated agriculture is a major basis of the regional economy. Virtually all of the water used for agricultural, commercial, municipal, and domestic purposes in the two-county area is groundwater withdrawn from the High Plains Aquifer within the Miocene- to early Pliocene-age (~20 to ~5 million years old) Ogallala Formation. This study characterizes current conditions and changes in groundwater levels since the 2004–2007 period, and describes variations in groundwater chemistry across the region. These data are then discussed in the context of the extensive, ongoing withdrawals of groundwater, physical and chemical processes controlling the water quality and chemistry, and possible sources and physical mechanisms of recharge to the aquifer.
The long-recognized importance of the High Plains Aquifer and the Ogallala Formation as a groundwater source has resulted in an enormous number of geologic and hydrologic studies. The geology of the aquifer has been characterized and the bedrock surface at its base has been mapped in detail. Thousands of water level measurements since the 1930s have documented a progressive and ongoing decline in groundwater levels due to decreasing volumes of water in storage. Storage is decreasing because groundwater withdrawals continue to greatly exceed recharge.
Many studies have focused on quantifying the amount of recharge to the High Plains Aquifer and attempted to identify spatially where it may occur. In general, recharge can vary greatly in space and time and is an inherently difficult quantity to measure. Estimated recharge quantities, in units of inches or millimeters per year, have ranged widely. However, there is a consensus that what natural recharge does occur is dominated by infiltration though playas of accumulated precipitation. Regardless of the amount of recharge, it has been and continues to be much less than the amount of water withdrawn from the aquifer by pumping.
In this study, water levels measured in 121 wells from 2010–2015 in Curry and Roosevelt counties were compared with water levels from 2004–2007 in the same wells. Thirty-four water samples were collected from public supply, irrigation, and domestic wells in summer 2015 and analyzed for major ion chemistry, trace elements, and the stable isotopic composition of oxygen and hydrogen. Eleven of the samples were also analyzed for the environmental tracers tritium and carbon-14 to aid in understanding groundwater recharge. Historic water chemistry data were obtained from the U.S. Geological Survey and the New Mexico Environment Department.
Aside from progressively declining water levels, current (2010–2015) groundwater conditions are similar to those in 2004–2007. Regionally, groundwater flows east and southeast, except where flow is diverted into northwest-southeast trending groundwater troughs. These coincide with paleochannels eroded into bedrock at the base of the Ogallala Formation. Very low water table gradients result in slow groundwater flow velocities and long travel times. Based on the depth to water, some of the large playas appear to be sites of groundwater discharge. The maximum depth to water of greater than 450 ft occurs north of Clovis. The depth to bedrock and the ground surface elevation are the main regional controls on the depth to water.
Declines in the thickness of the saturated portion of the aquifer since 2004–2007 are due to the long-term trend of groundwater withdrawals greatly exceeding recharge. In some areas, the High Plains Aquifer has been dewatered down to the underlying bedrock. The median water level decline was 4.2 ft from 2004–2007 to 2010–2015. Ninety-one of the 121 wells experienced net water level declines and 30 experienced net rises over the time interval reviewed.
The apparent change in the volume of water in the aquifer from 2004–2007 to 2010–2015 is a loss of 1,943,105 acre-feet. The average apparent net change in water volume over the study area is a loss of 277,586 acre-feet per year. Spatially delineated yearly apparent losses and gains of water in the study area correlate reasonably well with independent estimates of groundwater withdrawals and recharge. This correlation is quite tentative however, due to the limited amount of well data, and the numerous assumptions required for these independent estimates to be considered as equivalent. A conservative interpretation of the water level data is that the estimates of groundwater withdrawals (discharge) are generally in accord with independent estimates, groundwater withdrawal estimates are much more robust than the estimates of recharge, and that withdrawals continue to be several times the amount of recharge, resulting in net losses of water in storage.
Water samples from five wells had concentrations of several chemical constituents that exceeded maximum contaminant levels recommended for drinking water. Samples from three of these wells also exceeded some secondary drinking water recommendations for other constituents. Concentrations of other analyzed chemical constituents in these and the remaining samples are within acceptable ranges for drinking water.
Water chemistry shows regional differences, with homogeneous chemistry north of the Portales Valley, and great variety to the south. Processes affecting water chemistry may include dissolution of solutes in the soil and unsaturated zone, evapotranspiration of recharge water prior to infiltration, reaction with aquifer materials during groundwater flow, and mixing with water derived from bedrock units beneath the Ogallala Formation. Evaluating the relative importance of these processes is difficult, and they probably vary spatially. There have not been large changes in groundwater chemistry since the 1950s.
Water chemistry and environmental tracer data are consistent with some recharge occurring via return of irrigation water to the aquifer. This is not “new” water added to the aquifer, but rather a return of some of the groundwater previously withdrawn for agricultural use. There is no evidence of significant recharge occurring via infiltration of precipitation though playas, although this process has been shown to be the main recharge mechanism to the High Plains Aquifer in previous studies. Irrigation return can introduce agricultural chemicals, pesticides, and other contaminants to the aquifer. Irrigation is widespread in the study area, and thus introduction of contaminants may be also. Given the regional importance of playas, their protection as potential recharge sources should be considered, and efforts made to keep them free of contaminants that could infiltrate to the aquifer along with any recharge water.
The data and interpretation in this study are consistent with many other studies throughout the southern High Plains, and indicate that groundwater withdrawals continue to greatly exceed recharge. The result is progressive declines of the quantity of groundwater in storage, resulting in water level declines.
With regard to the protection of the source water for the Curry and Roosevelt County region, the groundwater level declines indicate a concern for groundwater availability in the region. There is evidence of naturally occurring groundwater contaminants, such as arsenic and fluoride. Alternative groundwater options are limited in the area, as aquifers in the underlying bedrock have poor water quality, and limitations to pumping. There are no significant surface water resources. Addressing both water quantity and water quality concerns through increasing public awareness and education, with particular focus on irrigation practices, may help improve the situation. However, long-term, drastic water conservation measures across the broader region may be the most effective means of extending the useful life of the High Plains Aquifer.
Funding was provided by the New Mexico Environment Department, Drinking Water Bureau, as part of the Source Water Protection program. Additional support was provided by the New Mexico Bureau of Geology and Mineral Resources and its Aquifer Mapping Program.
Results
- Rawling, Geoffrey C., 2016, A hydrogeologic investigation of Curry and Roosevelt counties, New Mexico, New Mexico Bureau of Geology Mineral Resources, Open-file Report 580, pp. 47.
Related Publications
- Rawling, Geoffrey, 2018, Mapping the Lifetime of the Ogallala Aquifer in East-Central New Mexico, New Mexico Earth Matters, v. 18, no. 2, pp. 1-6.
- Rawling, Geoffrey C.; Rinehart, Alex J., 2018, Lifetime projections for the High Plains Aquifer in east-central New Mexico, New Mexico Bureau of Geology Mineral Resources, Bulletin, v. 162, pp. 1-47.
- Rawling, Geoffrey, and Timmons, Stacy, 2017, Mapping the Aquifer Lifetime in the Curry and Roosevelt County Region, New Mexico Bureau of Geology & Mineral Resources, Aquifer Mapping Program Fact Sheet, 4 pp. (this is a nontechnical summary of the report for the general public).