Borehole Environmental Tracers for Evaluating Net Infiltration and Recharge through Desert Bedrock

Abstract

Permeable bedrock aquifers in arid regions are being increasingly developed as water supplies, yet little is generally known about recharge processes and spatial and temporal variability. Environmental tracers from boreholes were used in this study to investigate net infiltration and recharge to the fractured Navajo Sandstone aquifer. Vadose zone tracer profiles at the Sand Hollow study site in southwestern Utah look similar to those of desert soils at other sites, indicating the predominance of matrix flow. However, recharge rates are generally higher in the Navajo Sandstone than in unconsolidated soils in similar climates because the sandstone matrix allows water movement but not root penetration. Water enters the vadose zone either as direct infiltration of precipitation through exposed sandstone and sandy soils or as focused infiltration of runoff. Net infiltration and recharge exhibit extreme spatial variability. High-recharge borehole sites generally have large amounts of vadose zone tritium, low chloride concentrations, and small vadose zone oxygen-18 evaporative shifts. Annual net-infiltration and recharge rates at different locations range from about 1 to 60 mm as determined using vadose zone tritium, 0 to 15 mm using vadose zone chloride, and 3 to 60 mm using groundwater chloride. Environmental tracers indicate a cyclical net-infiltration and recharge pattern, with higher rates earlier in the Holocene and lower rates during the late Holocene, and a return to higher rates during recent decades associated with anomalously high precipitation during the latter part of the 20th century. The slightly enriched stable isotopic composition of modern groundwater indicates this recent increase in precipitation may be caused by a stronger summer monsoon or winter southern Pacific El Niñ o storm track.

Controls on the variability of net infiltration to desert sandstone

“…It is assumed therefore that the Sc values of the excavations represent peak concentrations of solute accumulation. Net infiltration rates determined previously by using the vadose zone 3 H depth-to-peak method [Cook et al, 1994] ranged from 2 to more than 57 mm/yr at 11 borehole locations in Sand Hollow [Heilweil et al, 2006] (Figure 3). Dividing the recharge rate at each location by the precipitation rate yields the average annual net infiltration ratio.…”

“…Although the spatial distribution of the GIS-predicted rates is based on vadose zone solutes (Cl, Sc), net infiltration values are determined by correlations between solute accumulation and borehole vadose zone 3 H using the depth-to-peak method rather than the Cl mass balance method. This approach was designed to take advantage of the much larger data set of solute concentrations along the 3350 m of excavations for describing the relative spatial variability in net infiltration, yet the more precise rates (±50%), primarily because of uncertainty in atmospheric Cl deposition rates, and (3) 3 H-based net infiltration rates at high-recharge borehole sites in Sand Hollow more closely correspond to groundwater Cl-based estimates than rates based on vadose zone Cl [Heilweil et al, 2006]. In this previous work, recharge rates determined with the CMB method using water table Cl concentrations were compared to net infiltration rates determined from vadose zone borehole environmental tracers using (1) the CMB method with Cl from the soil/bedrock interface to the bottom of the Cl bulge, (2) the CMB method with Cl from the bottom of the bulge to the water table, (3) the 3 H depth-to-peak method.…”

“…Portions of the core samples also were weighed and then oven dried at 105°C for 24 hours to determine gravimetric water content. Gravimetric water content was converted to volumetric water content by using a bulk density representative of the Navajo sandstone at Sand Hollow of 1,980 ± 110 kg/m 3 [Heilweil et al, 2006].…”

Global synthesis of groundwater recharge in semiarid and arid regions

“…Recharge rates range from 2 to 57 mm year 1 according to unsaturated zone tritium data, and from 0Ð5 to 13 mm year 1 according to unsaturated zone chloride data beneath the bulge. Recharge rates based on saturated zone Scanlon et al, 2005a) chloride concentrations are similar (mean ¾11 mm year 1 , ¾5% of precipitation, range 3-60 mm year 1 ) (Appendices I, II; Table I; Heilweil et al, 2006). Noble gases and tritium-helium dating in the saturated zone were used to estimate recharge in eastern Salt Lake Valley in northern Utah (Manning and Solomon, 2004 adjacent Wasatch Mountains represents ½30% and more likely 50 to 100% of recharge throughout the basin.…”

“…Studies have been conducted in Argentina in areas where pampas grasslands have been replaced by eucalyptus plantations Figure 12. Relationship between recharge and precipitation from studies of large natural areas 40-374 200 km 2 using methods that reflect regional recharge rates (modelling, saturated zone CMB, micro-gravity, and water-table fluctuations) (Leaney and Allison, 1986;Edmunds et al, 1988;Bazuhair and Wood, 1996;Sami and Hughes, 1996;de Vries et al, 2000;Love et al, 2000;Anderholm, 2001;Leduc et al, 2001;Flint et al, 2002Flint et al, , 2004Favreau et al, 2002b;Harrington et al, 2002;Hevesi et al, 2003;Goodrich et al, 2004;Sanford et al, 2004;Heilweil et al, 2006;Keese et al, 2005) Copyright (Jobbagy and Jackson, 2004). Detailed studies of the impact of a 40-ha eucalyptus plantation on groundwater over 2 years indicated that groundwater discharged more than 50% of the days through ET depressing the water table by >0Ð5 m and increasing groundwater salinity by factors of 2-19, depending on soil texture (Jobbagy and Jackson, 2004;Engel et al, 2005).…”