SCERP Project Number: P-18
Principal Investigator: Duncan T. Patten
Arizona State University
Abstract: The Santa Cruz River is an international river of great
ecologic and economic importance. Much of the lower SCR near Tucson is
ephemeral and devegetated because of groundwater pumping, but the upper
SCR remains a free- flowing desert river. Groundwater is pumped from beneath
the floodplain of the upper SCR to supply water to Nogales, Arizona and
other municipalities; but a large portion is returned to the river as effluent
from the Nogales International Waste Water Treatment Plant (IWWTP). In
this same area raw sewage is periodically released into the river via Nogales
Wash. The effluent has enriched stream nutrient levels and raised riparian
water tables. These conditions, together with periodic flood flows, have
allowed for the development of a dense, rapidly growing riparian forest
of Populus fremontii and Salix gooddingii and of a more diverse aquatic
biota downstream of the IWWTP. The forest, in turn, is contributing to
the reduction in downstream nutrient and pollutant concentrations. Upstream
of the IWWTP near municipal well fields, there are local areas of water
table depression and reduced riparian vegetation abundance. These factors
highlight the need to consider basin-wide ecological impacts of groundwater
extraction and effluent release. Attention should be paid to the idea of
establishing effluent discharge point(s) immediately downstream of the
main area(s) of municipal water extraction, to compensate for adverse impacts
of well-fields on riparian and aquatic biota.
INTRODUCTION
Many desert rivers in Arizona have been dewatered. The lower reaches of the Salt River, Gila River, and Santa Cruz River (SCR) for example, now have ephemeral flow as a result of upstream surface water diversion or groundwater mining. Flow in many others, including the Bill Williams, Verde, and Colorado, has been reduced and regulated by upstream dams. Manipulation of water resources has resulted in loss or degradation of many of the native riparian and aquatic ecosystems associated with lowelevation rivers of the Southwest. As water use in Arizona shifts from agricultural to municipal use, however, the riparian and aquatic ecosystems along several river reaches are being "rejuvenated" by discharge of effluent from municipal and industrial waste water treatment plants. This has been the case for, among others, the Salt River below Phoenix's 91st Avenue sewage treatment plant; the lower SCR below Tucson's Roger Road and Ina Road sewage treatment plants; and the upper SCR downstream from the Nogales International Waste Water Treatment Plant (IWWTP) (Sullivan, 1991).
There are many unresolved legal and ecological issues regarding the release of effluent into arid region stream beds (Tellman, 1992). For example, how does increased nutrient content of the effluent affect composition and abundance of aquatic and riparianbiota? What are the pollutant loading rates above which biotic community structure and function are degraded? What is the role of biota in reducing downstream nutrient and pollutant concentrations to levels in accordance with Federal Clean Water Act standards? How do potential ecological consequences of pollutant or nutrient loading compare to the consequences of water reduction caused by removing effluent from the stream bed? What role does riparian vegetation play in enhancing percolation of the effluent into the aquifer?
The SCR is an international river of great ecologic and economic importance to the US/Mexico border region near Nogales. Much of the lower SCR near Tucson is ephemeral and devegetated because of groundwater pumping. The upper Santa Cruz, however, retains many natural characteristics and has been identified as a river of high ecological significance by the Arizona Nature Conservancy. Groundwater is pumped from the aquifer underlying the upper SCR to supply water to the border cities known as Ambos Nogales and to other municipalities. Some of this water is returned to the river as effluent from the Nogales International Waste Water Treatment Plant (IWWTP). In this same area raw sewage is periodically released into the river via Nogales Wash, because many homes in Nogales, Sonora are not connected to sewage disposal systems.
In 1992 we initiated a multi-year ecological study of the upper SCR
and its riparian and aquatic zones. Objectives of the first year were to
characterize and map abiotic and biotic components of the SCR riverine
system, including water quality, ground and surface water hydrology, riparian
vegetation, and aquatic biota. Objectives of the ongoing second year study
are to understand the functional relationships between the various abiotic
and biotic components of the riverine system. Results in this paper focus
on ecological issues related to the extraction of alluvial groundwater
(i.e., SCR underflow) and release of effluent from the IWWTP into the SCR
bed.
The FY92 SCERP-supported phase of this project: P-18
The FY93 SCERP-supported phases of this project:: WQ93-12
and BEAT
Last updated 7/1/99