— May 28, 2019
An aquifer can be considered like a bank account. The deposits or credits typically consist of natural recharge adding water to the aquifer (like precipitation or river water seeping into the ground and reaching the groundwater table). Withdrawals take water out of the aquifer, and can include discharge into rivers or pumping of wells. Most cities are concerned with the withdrawal side of the equation and hope nature takes care of the deposits. But Albuquerque has undertaken the progressive measure of inputting additional recharge (deposits) now so there will be sufficient water for future withdrawals, something called managed aquifer recharge (MAR). To that end, the Albuquerque and Bernalillo County Water Utility Authority (ABCWUA) has recently completed a well for deep injection of excess river water into the aquifer, and is currently running surface water down the upper part of Bear Canyon Arroyo for near-surface recharge.
To help the ABCWUA plan where to conduct MAR in the near future, the New Mexico Bureau of Geology has recently finished two maps showing suitable areas to conduct MAR. One map shows suitability of MAR via deep-injection wells and the other via shallow-based methods (like arroyo-bottom recharge, infiltration basins, infiltration galleries, or shallow wells in unsaturated sediment). These maps were constructed using the weighted overlay method in ARCGIS and are at a scale of 1:36,000. Criteria that went into the analyses include geologic properties of the aquifer that relate to groundwater flow and storage, infiltration characteristics of the soil, the slope of the water table, depth to groundwater, and water infrastructure (e.g., pipelines, wells). MAR suitability is illustrated via color-shading and hatching that depict areas interpreted to be Unsuitable, Poorly Suitable, Suitable, and Most Suitable. A complementary map shows the susceptibility of the region to soil hydrocompaction, which is collapse phenomena of the ground surface that occurs after saturation of certain types of soils (having initial high porosity and composed of clayey sand that was rapidly deposited and never fully wetted).
We used weighted overlay analyses to map suitability for managed aquifer recharge (MAR) in the eastern Albuquerque metropolitan area. The study area extends from the Rio Grande eastward to the Sandia Mountains, and from Sandia Pueblo southward to ~2 km south of Tijeras Arroyo. This area falls under the jurisdiction of the Albuquerque Bernalillo County Water Utility Authority (ABCWUA), who would likely be the main user of this map. The subsurface Santa Fe Group stratigraphy consists of Rio Grande axial-fluvial sediment that interfingers westward with the Rio Puerco distributive fan system and eastward with piedmont sediment from the Sandia Mountains. The Santa Fe Group is overlain by up to 51 m of weakly consolidated middle to late Quaternary piedmont alluvium, Rio Grande terrace deposits, and valley fills. Long-term pumping by the city has created a large, trough-like cone of depression centered in the study area, with up to 500 ft (150 m) of unsaturated, relatively permeable sediment that could be used to store excess surface water allotted to ABCWUA from the San Juan-Chama Drinking Water Project.
We produced two MAR suitability maps with a grid cell resolution of 100x100 m: one showing the suitability for deep (saturated zone) injection recharge and the other for shallow (infiltration or vadose zone injection) recharge. Unsuitability exclusionary buffers were a priori assigned to the Rio Grande floodplain (due to potential for injected water to reach the river or induce swamping) and 1/2 mile around known groundwater contamination sites. For the deep-injection MAR suitability map, an exclusionary buffer zone was also drawn along major faults. Initial steps included: (1) inspecting outcrops to qualitatively assess the permeability character of lithologic units that extend beneath the study area; (2) compiling hydraulic data from pump and infiltration tests; (3) drawing structural contours of 10 lithologic units under and near the study area; (4) assessing the proportions of sand, clayey sand, and clay layers for these units (primarily using interpretation of wireline logs of 17 deep wells); and (5) utilizing ArcGIS tools to construct a 3D geologic model. For the weighted overlay analyses, we considered several criteria that could impact MAR. For deep injection recharge, these include transmissivity, the typical storage zone thickness (thickness of permeable beds between clay layers), allowable injection rates, water table gradient, density of ABCWUA and non-ABCWUA wells, and distance to existing water pipelines. For shallow recharge, criteria include surface soil characteristics (hydraulic conductivity and drainage classes from Natural Resources Conservation Service [NRCS] soil maps), surface slope, depth to groundwater, percolation time to reach the water table, and the proportion of clay layers. Each criterion were subdivided into classes (binned), which were ranked from 0 to 2 based on their impact to MAR (2 being most favorable and 0 being least). For criteria that vary with geologic unit (e.g., transmissivity, storage zone thickness, and allowable injection rates), each individual geologic unit’s score was weighted for their relative thickness in the 1500 ft-deep zone of interest, summed, then normalized to between 0 and 2 at each grid cell location to produce a single score. All criteria at each cell were then weighted (using best-judgment by the authors and ABCWUA), summed, and normalized (to 0-2) to produce an overall rating. We compared the overall ratings to previous maps of known or hypothesized locations of MAR-suitable sites, and then used histogram analysis to translate the scores to qualitative MAR suitability ratings. If water is spread on the ground for the purposes of shallow recharge, then soil hydrocompaction is a valid concern in the Albuquerque area. Consequently, we produced a complementary map showing the susceptibility of regions within the study area to soil hydrocompaction. This map was also made using weighted overlay analyses of correlative susceptibility factors. These included soil taxonomy from the order down to the great group level, soil texture from Soil Survey Staff (2014) sources, depth-to-water derived from the water elevation contours presented in this report, and geologic unit as mapped in Connell (2008).
The resulting MAR maps depict three color-coded suitability bins as well as exclusionary zones: poor-suitability, suitable, and most-suitable. For deep-injection recharge, the most suitable areas extend north-south in two areas: between Interstate 25 and Moon Street and near Tramway Boulevard south of Montgomery Boulevard. Poor suitability lies under the Rio Grande floodplain (corresponding to an exclusion zone) and near Eubank and Juan Tabo Boulevards north of Candelaria Road. For shallow-based recharge, the central part of the study area is most suitable, extending northwest past Interstate 25 to the Rio Grande floodplain. Poorly suitable areas include: (1) north and south of lower Tijeras Arroyo, (2) most of the floodplain south of Ranchito Road, and (3) a north-south swath between Moon Street and Tramway Boulevard. The eastern poor-suitability zone in both maps corresponds to a subsurface geologic unit of relatively low permeability (distal piedmont and its interfingering zone with axial-fluvial deposits). Most suitable areas in both maps commonly correspond to thick axial-fluvial deposits of the Rio Grande and the medial-proximal piedmont.
The hydrocompaction map indicates high-extreme susceptibility for much of the area north of Interstate 40 (especially near Interstate 25 north of Comanche Road), in the eastern Rio Grande floodplain (west of Interstate 40 around Edith Blvd.), and in the bottom of Tijeras Arroyo. Site-specific geotechnical studies should be conducted if groundwater spreading is to be conducted in areas noted as high or extreme on this map.
The work is funded by the ABCWUA and conducted by Dan Koning (P.I.), Colin Cikoski, Andy Jochems, and Alex Rinehart. The results will released as an open-file report in June or July of 2019.