ADWR completes groundwater flow model update of North Santa Cruz AMA

5.10.2019 Santa_Cruz_River

Photo courtesy of University of Arizona Water Resources Research Center

Arizona Department of Water Resources hydrologists have completed an update  to the North Santa Cruz Active Management Area groundwater flow model, the first such updates since the release of modeling reports on the NSCAMA in 2007 and 2010.

The model is updated to include data for the approximately 14-year period beginning in 2002 through water year 2016. The calibrated, extended model will be used to provide model-simulated estimates of natural recharge and discharge components for the draft Fourth Management Plan for the Santa Cruz AMA.

Overall, the NSCAMA model update did not produce any big surprises or changes in the Department’s interpretation of the hydrology in  the area.

However, the groundwater flow model update did reinforce the importance to the NSCAMA aquifer of “episodic flood pulses” — episodes, usually lasting days or weeks, when runoff from large rainfall events flows into the Santa Cruz River and increases the amount of surface flow, often by orders of magnitude.

Water levels in the Santa Cruz AMA are largely dependent on stream recharge, which varies significantly from year to year in response to streamflow coming down the Santa Cruz River. That recharge mostly occurs as a result of those episodic flood pulses generated by substantial rain events.

The North Santa Cruz AMA aquifer is a narrow, shallow basin that provides less long-term storage capacity than wider, deeper aquifers such as that of the Tucson and Phoenix AMAs. Especially following major rain events, water flows through the aquifer quickly because the soil properties are such that the conductivities are very high.

The model update found that groundwater pumping in the southern Tucson AMA is continuing to impact water levels in the northern Santa Cruz AMA north of the town of Tubac.

From 1997 to 2016, water levels in this area have been steadily dropping – up to 45 feet, using 1997-1998 as the baseline.

The northern portion of the Santa Cruz AMA is simulated as a model separate from the southern portion due to the  distinct hydrologic regimes along the upper and lower reaches of the Santa Cruz River within the AMA.

For further information regarding the model update of the North Santa Cruz AMA, contact Sally Stewart Lee at ADWR.




Water-Treatment Plant Owner Plans Public Hearing On Threats To Santa Cruz River

Nogales sewage

For the people of Nogales, Arizona, the sight of an approaching storm has become something to dread.

Water pouring onto the drought-parched desert of southern Arizona is a wonderful thing. All that water flowing into – and overflowing out of — the fast-disintegrating waste-water sewage system the community shares with Nogales, Mexico, is the stuff of nightmares. Rapidly worsening nightmares that may cost tens of millions of dollars to properly fix.

As a part of the long-running effort to resolve the sewage issues, the co-owner of the area’s sewage-treatment facility, has announced plans for a public meeting in the town of Tubac on September 13.

The purpose of the forum, sponsored by the U.S. Section of the International Boundary and Water Commission, or USIBWC, will be to promote the exchange of information between the USIBWC and the community regarding Commission projects and related activities in Pima, Cochise, and Santa Cruz counties. Specifically, the discussion will air issues regarding source metals that are being found in Nogales wastewater, as well as the health of the Santa Cruz River.

The Nogales sewage problems begin with a woefully over-taxed 8.8-mile sewage-drainage system known as the International Outflow Interceptor, or “IOI.”

At times, especially during storms, millions of gallons of untreated sewage have been spilling out of breaks in the nearly 70-year-old IOI and elsewhere in the system. In July 2017, the IOI ruptured under the strain of storm water surging up from Mexico, which, as reported by the Arizona Daily Star, spilled “raw sewage into a tributary of the Santa Cruz River and [caused] a significant spike in E. coli bacteria levels near the breach.”

And not just human waste, either. Untreated industrial wastes from Nogales, Mexico – which include regulated mater­­­­ials such as cadmium, lead, copper and zinc — have been discovered in “significant levels” in the wastewater. The pollutants are contaminating the Nogales Wash, under which the IOI is buried, as well as water leaching into the Upper Santa Cruz aquifer and the Santa Cruz River itself.

Health officials, including those from the Arizona Department of Environmental Quality, now believe that the overtaxed wastewater system is becoming a serious environmental threat to the health of the river, to say nothing of the thousands of Arizonans that rely on groundwater wells tapping into the area aquifer.

The Santa Cruz relies heavily on treated effluent from the Nogales International Wastewater Treatment Plant, or NIWTP, near Rio Rico. Co-owned by the U.S. Boundary and Water Commission and the City of Nogales, Arizona, the plant is designed to treat nearly 15 million gallons of water daily, which accounts for about 38 percent of the Santa Cruz flow at that point.

Roughly 10 million gallons of that daily capacity are allocated to Nogales, Sonora, a much larger community (pop. 212,500) than Nogales, Arizona (pop. 20,000). On the U.S. side, Nogales and Rio Rico are allocated 4.84 million gallons of capacity. Annually, Nogales, Arizona, uses just 12 percent of the IOI system providing the as-yet untreated sewage to the plant, while the vast majority of the rest flows north across the border from Sonora.

At the September 13 public meeting in Tubac, a hydrologist from the Arizona Department of Environmental Quality will talk about metals in wastewater treated at the NIWTP.

The meeting also will include a presentation by representatives of the Sonoran Institute on the health of the Santa Cruz River.

Who: The International Boundary and Water Commission United States Section

What: A southeast Arizona citizens forum

When: Thursday, September 13; 3 – 5 p.m.

Where: Tubac Community Center; 50 Bridge Road; Tubac, Arizona 85646