EBTAG Annual Workshop and Field Trip
October 12-13, 2017


What is Impairment?

Tom Morrison

P.E., Groundwater Consultant, Santa Fe, NM, 87501, tom.morrison@state.nm.us

State law requires a permit from the Office of the State Engineer (OSE) in order to drill a water well in a declared underground water basin. To obtain a permit an application must be filed with the OSE. Non-domestic applications (municipal, agricultural, industrial, and commercial uses) are evaluated by the OSE to determine whether the proposed use will impair existing water rights. A permit cannot be issued if a proposed use impairs existing water rights.

Impairment is a finding by the state engineer of excessive negative impact to existing water rights and may be caused by increased stream depletion on a fully appropriated stream and excessive groundwater level decline on existing wells. The OSE has developed procedures to evaluate water level decline estimates to determine whether they are excessive.

The procedures involve two types of impairment analysis: local assessment and regional assessment. For the local assessment two types of limits (economical and physical drawdown constraints) are determined for existing wells in the vicinity of a proposed well to determine the allowable water level decline each well may tolerate. The economic drawdown constraint is equal to 70 percent of the initial water column in a well while the physical drawdown constraint is determined as the difference between the lowest practical pumping level and the current static water level. The most conservative constraint, the one giving the shortest water column, is selected for further analysis.

Once the drawdown constraint is obtained it is compared to the total predicted 40-year drawdown on each nearby well. The total drawdown is the sum of three components: 40-year water level decline due to the exercise of existing water rights, 40-year water level decline due to the proposed use, and the self-induced drawdown as pumps are cycled on and off. If the total drawdown exceeds the drawdown constraint the well is classified as a “Critical Well” since it has less than a 40-year life.

The next step is to evaluate the magnitude of the drawdown on the critical wells caused by the use of a proposed well. The drawdown is compared to the drawdown allowance which defines the small incremental drawdown which may be permitted. A drawdown may be deemed excessive if the incremental drawdown on a critical well exceeds the drawdown allowance. However, before an impairment determination may be made, an evaluation of whether the critical well is reasonably completed is performed. In addition, other factors such as well age and whether the affected well may be deepened are taken into account.

Regional assessments may be performed for basins where a regional groundwater flow model is available. The methods are similar to the local assessment procedures except average well completion characteristics are used to obtain the regional drawdown constraint. Areas in which the total drawdown exceeds the drawdown constraint are designated as Critical Management Areas. A drawdown allowance is also applied to determine excessive drawdown.

15th Annual Espanola Basin Technical Advisory Group Workshop and Field Trip
October 12-13, 2017, Santa Fe Convention Center