Heat map of the percentage contribution of vadose water to a floodplain aquifer studies near Rifle, Colorado, based on Strontium isotopic and concentration data from the vadose zone and groundwater. White circles represent locations of sampled monitoring wells used in the study.

Berkeley Lab researchers who study aquifer recharge and groundwater quality measured concentrations and isotopic composition of the trace element Strontium (Sr) in groundwater to evaluate how local topography affects the amount of groundwater recharge across a semi-dry riparian floodplain in the Upper Colorado River Basin. Their study location was the Rifle Site, one of 18 former uranium-mill tailings sites near Rifle, Colorado. The site became home to studies into groundwater remediation when Congress-ordered efforts to clean up the sedimentary contaminants known as U-mill tailings resulting from uranium mining concluded in the late 1990s.

The Rifle Site is now the subject of research related to the Watershed Function Scientific Focus Area at Berkeley Lab. Staff scientist John Neil Christensen led a team to examine the connection between the unsaturated — or vadose — zone — and groundwater quality. They specifically looked at the groundwater recharge process, or the movement — or drainage — of water from land surface to the aquifer. Results of their research were published here. 

“The vadose zone is important not just to water quality but also to whether there will be sufficient amounts of available groundwater,” says Christensen. “We’re interested in learning how much water passes through the vadose zone so that we can understand the transfer of nutrients and contaminants between the earth’s surface and groundwater.

“The Rifle Site is not only relevant to our studies of groundwater because of the previous contamination from ore processing, its location in a semi-arid region also makes it an ideal testing ground for studying groundwater availability.”

Because the Rifle Site has been the subject of so much research, Christensen’s team had access to a number of groundwater monitoring wells for their research. “I was looking at porewater samples of groundwater from above the vadose zone taken from the wells,” he says. The team then estimated quantities of groundwater from the vadose zone to the aquifer at various spots across the site using a Sr mixing model between groundwater and vadose porewater to interpret the the variation of groundwater Sr isotopic composition as the variable contribution of vadose zone porewater.

“The Rifle Site is not only relevant to our studies of groundwater because of the previous contamination from ore processing, its location in a semi-arid region also makes it an ideal testing ground for studying groundwater availability.”

The vadose zone porewater contribution ranged from 0 percent to as much as 38 percent and appears to be distinguished by the site’s microtopography.  Slightly elevated areas of the site are associated with essentially zero recharge and hence relatively low rates of mobility of dissolved chemicals within the vadose zone.

Because total elevation across the Rifle Site varies minimally by an approximate 2.7 meters, lower-lying areas are the focus of drainage and greater amounts of recharge and, therefore, increased transfer of dissolved material to groundwater. The results of Christensen’s studies averaged over the site, and combined with average rainfall, suggest a water loss of 83 to 92 percent from the vadose zone due to evapotranspiration, resulting in an average estimated groundwater recharge rate of 2.5 to 5 cm per year.

This study provides a new set of tools for measuring the recharge of aquifers in semi-arid regions — both for groundwater resource management and evaluation of processes of vadose communication with groundwater both in terms of nutrients affecting biological activity and contaminants affecting water quality.