SIERRA VISTA — Arizona Department of Water Resources completed the San Pedro basin well sweep study, and staff hydrologists found decline of water levels in more than 77 percent of the wells tested since 2007.
The results of the 2019 study have been added to ADWR’s “Water Level Change Map Series” which is available to the public on the website. “The map series is designed for a rapid public release to provide hydrologic information and data with only limited interpretation,” according to the website. “The report includes a map and a general overview of water levels and groundwater conditions within the basin.”
This latest study covers the San Pedro basin where water has been a concern for years and includes the San Pedro Riparian National Conservation Area and the towns of Sierra Vista, Huachuca City, St. David, Benson, Tombstone and Bisbee.
Staff hydrologists measure the depth to groundwater in ADWR, U.S. Geological Survey and wells volunteered by landowners across the basin.
ADWR staff collected 764 water level measurements in 2007 and 523 in 2019, which include index wells monitored by ADWR on an annual basis to evaluate water level changes across the 12-year time frame.
The report shows considerable evidence of groundwater declines in many areas with an average decline of 3.9 feet overall in 77.5 percent of the wells tested. Depth to water remained unchanged in 3.7 percent of the wells, while 18.8 percent of the wells showed a rise in water levels.
“Many of the large water-level changes, including both increases and decreases, occurred near streambeds and mountain fronts, which are susceptible to large, short-term water-level fluctuations due to recharge events,” the report stated.
Holly Richter, Arizona Water Projects Director for The Nature Conservancy, said after a review of the study, “The fact that water level declines were detected over the 12-year period in 77.5 percent of the wells sampled in the Upper San Pedro Groundwater Basin is not good news for people, or nature. Yet, by looking at the ADWR map, it’s also clear that groundwater levels were stable, or even increasing, during this same time where additional water supplies were stored underground either through natural processes in the tributary drainageways at the Huachuca mountain fronts and in the floodplains of the San Pedro River or where water levels were enhanced as a result of the projects underway to actively replenish groundwater, and where historic pumping has been reduced to conserve groundwater supplies.
“This information only reinforces our need for additional support of future efforts to enhance the storage of water underground by protecting natural floodplains and drainageways and by the continued development of additional replenishment and water conservation projects.”
Well basin sweep study findings
Many of the large water level changes, both increases and decreases, occurred near streambeds and mountain fronts that are susceptible to large, short-term water level fluctuations due to recharge events, the report states.
Water level changes south of the Winchester Mountains range from a decline of 15.5 feet to a rise of 5.9 feet. Groundwater data in this area is limited due to a lack of available monitoring sites.
Water level changes along the San Pedro River north of Benson largely show water level declines, with values ranging from a water level decline of 23.8 feet to a water level rise of 1.2 feet. Most of the wells in this area are shallow and highly responsive to surface water conditions along the San Pedro River.
Wells in the vicinity of Benson show water level changes ranging from a water level decline of 18.6 feet to a water level rise of eight feet. Wells in this area are in both a shallow aquifer and the deeper artesian regional aquifer. A handful of wells in this area had water level remarks indicating flowing water conditions.
St. David generally displayed continued trends of water level decline, with the greatest observed water level decline of 11.9 feet. One index well used for annual readings has experienced continuous water level decline since monitoring began in 1944.
Following the San Pedro River south towards Fairbank, water level changes become less significant, with many wells showing limited water level change values. Two sites are situated between St. David and Fairbank and both have relatively stable water level trends.
The Tombstone area ranged from a decrease of 6.6 feet to an increase of 6.9 feet. One index well continues to show consistent water level decline with the depth to water dropping by more than 18 feet since measurements began in 1991.
In the Huachuca City, Sierra Vista and Nicksville areas water level changes ranged from a decline of 51.6 feet to an increase of 31.4 feet. Multiple readings in this area confirm sustained trends of historical ground water decline. However, a number of wells with water level rises were found along the foothills of the Huachuca Mountains.
Moving northeast from Sierra Vista toward Charleston, there are measured groundwater rises and declines. Water level changes ranged from a decline of 6.4 feet to a rise of 1.6 feet.
Wells indicating no change or positive water level change are concentrated along the San Pedro river near the SPRNCA.
High concentrations of water level decline were prevalent in the Hereford and Palominas areas. An exception is noted near the San Pedro River south of Hereford and north of State Route 92. Most of these wells are in the area’s deeper artesian aquifer system. Overall, water level change around Hereford and Palominas ranged from a water level decline of 14.7 feet to a water level rise of 5.4 feet. An index well located along the San Pedro River slightly east of Palominas exhibits a negligible water level decline.
Water level changes south of Bisbee to Naco indicate groundwater decline. Water level change ranged from a decline of 11.6 feet to less than one foot of change.