Spatial and Temporal Distribution of Bacterial Indicators and Microbial Source Tracking within Tumacácori National Historical Park and the Santa Cruz River Watershed

Science Center Objects

Elevated levels of bacteria have been measured by the USGS, National Park Service, Arizona Department of Environmental Quality (ADEQ), and other agencies, in the Upper Santa Cruz River, including within the reach at Tumacácori National Historical Park (TUMA).  Indicators of pathogens in the river at TUMA are at levels that are of great concern to TUMA personnel responsible for safeguarding two of the Park’s highest priorities, visitor health and the health of riparian and aquatic wildlife.   E. coli levels measured during base flow conditions exceed the full body contact standard and measurements during runoff events reach levels that are more than two orders of magnitude greater than the full body contact standard. Concentrations increase with increasing discharge and the Park is concerned about the potential hazard to TUMA visitors and personnel that come into contact with the water or recent flood deposits.


While some work has been done to identify sources of bacteria in the River, significant information gaps remain with regard to the location and source of bacteria. The primary source of surface water flow originates from the Nogales International Wastewater Treatment Plant (NIWTP; approximately 15 miles upstream of TUMA), but reports low-level E. coli concentrations in the effluent discharged from the outfall into the Santa Cruz River. The treated effluent from NIWTP appears to have a minor contribution to the fecal coliform and E. coli concentrations found within the watershed.  This finding indicates that the majority of E. coli loading is not originating from the NIWTP and that additional sources are contributing to the River further downstream. Samples have been collected along various reaches of the River, but sampling has been spatially and temporally inconsistent, and non-existent in ephemeral tributary washes. Suspended sediment and bacterial indicators will be collected from the mainstem and major tributaries during different flow conditions and seasons to better understand the spatial and temporal contributions of bacteria loading and sources. This data analysis will provide the Park with crucial information needed to protect the Park’s resources, personnel, and visitors, and to aid the Park in working with ADEQ to develop best management practices with neighboring properties. 

Map showing sampling locations and major contributing watersheds for the Tumacacori National Historical Park project.
Map showing sampling locations and major contributing watersheds near Tumacacacori National Historical Park, Southern Arizona.


The TUMA seeks to understand the spatial and temporal variability of bacterial indicators and identify sources of E. coli in order to provide much needed information to the Park staff, the public, and ADEQ. To accomplish this objective, spatial coverage will expand to the primary contributing watershed and include many parts of the mainstem as well as, major tributaries to the Santa Cruz River. Water and sediment samples will be collected on the Santa Cruz River mainstem and nearby tributaries at a frequency that captures the entire hydrograph on both a diel and a seasonal scale. Samples will be analyzed for suspended sediment, E. coli concentrations and analyzed for Bacteroides as method for source identification. Host-associated markers have been identified from different groups of fecal-origin bacteria, often from the genus Bacteroides, a bacterium abundant in the gut of warm-blooded animals and these can be used to identify and quantify bacterial indicators.  There will be a detailed characterization of land-use practices and septic tank distribution in the immediate adjacent sub-basins.


Relevance and Benefits

The results from the study will provide the Park, local groups, and state agencies with the necessary information and interpretation to implement cooperative strategies that focus on risk assessment and remediation efforts.  Source identification analyzed in relation to the spatial extent, loads, and land use activities will enable the Park and local groups to produce educational information that can be delivered through public forums and factsheets to the general public, water providers, and agricultural operations within the watershed. Without this information, TUMA cannot seek knowledge-based solutions to the ADEQ pending regulation of E. coli in addition to other known contaminant issues. In addition, without this information TUMA faces a high risk of harming wildlife, visitors, and the riparian environment in general because of a lack of understanding of water quality, contaminant sources, and variability through time. The investigation will help support TUMA with the possible opportunity to become a candidate for federal recognition through the National Heritage Area and the ability to pursue grants for land owners to improve conditions via the EPA’s Border 2020 Initiative or other avenues. Management options might include modifications to the wastewater treatment plant (if needed), filters (green belts), reduced grazing and agricultural activities or change in agricultural products used, increased inspection and regulation of industry in Nogales, Arizona and Nogales, Sonora.  Results of this study will also be valuable from a regional socio-economic perspective because of the proximity to Mexico.  The study will provide valuable information to other protected lands located within the larger watershed and aid efforts along the US-Mexico International Border aimed at understanding and mitigating sources of pathogenic contamination The data analysis will also help to improve a previously developed Soil and Water Assessment Tool (SWAT) model for the Santa Cruz River Watershed and help with ADEQ’s efforts in developing a Water Quality Division Total Maximum Daily Load (TMDL) for the River.