The Nature Conservancy has recorded Sonoita Creek surface water flow at its riparian preserve near Patagonia since 1992. Water flows from there into Patagonia Lake, joins the Santa Cruz River at Rio Rico, and flows north toward Tucson. The average flow has been about 3,500 acre-feet per year.
Mining company South32 is seeking permits for up to 7,000 acre-feet per year of treated dewatering effluent that will flow down Harshaw Creek from its treatment plant 900 feet above Patagonia. This would reduce mine tunnel flooding but triple Sonoita Creek’s total water flow. South32 estimates from their modeling that this effluent will raise groundwater levels in a wide area of the Sonoita Creek watershed, predicting benefits to residents.
Skepticism is warranted as local quantitative groundwater studies have so far not been done and so little is known about where Patagonia’s groundwater is coming from.
What do we know about groundwater in Patagonia?
Recently the authors made the first detailed analysis of depth-to-groundwater recorded at Patagonia’s municipal wells from 2007 onwards. Graph #1 shows these levels together with annual rainfall; groundwater depth numbers are plotted in reverse order for illustrative purposes and rainfall numbers are plotted in the middle of the corresponding year.
Clearly shown is a somewhat delayed response of groundwater depth to long-term rainfall behavior. Consistent with that is the 45-foot groundwater depth that caused some wells on Harshaw Road to go dry in 2014 because that event was preceded by a two-to-three-year period of below average annual rainfall.
Also, in spite of an essentially constant average annual rainfall of about 17 inches per year, the trend line of the groundwater graph demonstrates an annual increase of about 0.5 foot. This may relate to the threat of global warming where each year less rainwater is able to recharge the groundwater basin of Patagonia.
Valuable information about the short-term behavior of groundwater recharging by rainfall events was obtained from very high-resolution (96 times per day) groundwater depth recordings in a monitoring well at the Nature Conservancy. In the second chart, results for the 2019 summer monsoon season are presented, showing a very rapid return to the seasonal term trends. The oscillations observed are due to night-day variations in groundwater due to transpiration from nearby big trees.
The second chart correlates rapid rain events with rapid downhill groundwater flows. It can now be expected that with groundwater monitoring wells carefully sited at the intersections with major tributaries of Sonoita Creek, the sources of Patagonia’s water can be identified and floods from such events predicted.
What else is there to be concluded? The volume of the South32 effluent in Harshaw Creek is likely to saturate the deep, dry Sonoita Creek streambed soils just up SR-82 from Patagonia so that its capacity to absorb flash flood water will be reduced. This loss of groundwater recharge capacity may leave Patagonia exposed to major monsoon flash floods.
As shown above, saturation is going to happen quickly and with Patagonia taking only a small volume of groundwater for its residents, most of the effluent will be discharged in Patagonia Lake and therefore permanently lost for future recharging needs upstream.
It is also very likely that Sonoita Creek will flow perennially above its streambed at the town’s wells and at the Tucson Audubon Society stream crossing.
Editor’s note: Dave Ellis, of Patagonia, holds a PhD in Electrical Engineering from the University of Washington in Seattle. Ellis is a member of the Patagonia Flood & Flow Committee. Chris Werkhoven holds a PhD in Physical Chemistry from the University of Amsterdam.