2.3. Water quality methods

Surface water samples were collected from n = 7 sites in the Tijuana River canal during five sampling campaigns in June, July and August 2019 (Fig. 2). Two water sampling sites were from the Tijuana River directly upstream and downstream of the El Bordo population (sites RU and RD), three sites were active flows from storm drains into the canal (sites D1–3), one site was a riverside pressure-release tube – lloradero – (site RT), and one site was an outdoor tap of a convenience store (site ST). The latter two sites were selected due to ethnographic data indicating that they were frequently used by the study population.

Tijuana-San Diego Border Region. a. Tijuana-San Diego Border Region. b. Tijuana River map and water quality sampling sites: river upstream (RU), river downstream (RD), storm drain (D1–3), riverside tube, lloradero (RT), and store tap (ST).

We measured biophysical parameters, levels of fecal indicator bacteria [Fecal Indicator Bacteria - FIB; coliforms and Escherichia coli – E. coli] and chemical compounds found in wastewater. At the river sites RU and RD, water depth and turbidity [60 and 120 cm Transparent Turbidity Tube with Secchi Desk] were measured. Water temperature, pH, total dissolved solids (TDS), and dissolved oxygen (DO) were measured at all sites using a multiparameter sonde [Eureka Manta 2, Austin, TX] calibrated before each sampling campaign. All samples for bacteria and chemical testing were stored in sterile PET sample vessels or amber glass bottles rinsed beforehand with HCl and on-site with three volumes of sampling site water. Samples were stored at 4 °C until analysis. Total coliform and E. Coli [serial-diluted up to 10,000-fold] were enumerated using Colilert-18 kits within 4 h [IDEXX Laboratories, Westbrook, ME] for samples collected from sites RU, RD, RT, and SP. Samples were syringe-filtered within 8 h and diluted up to 50-fold prior to testing within 24 h by standard USEPA methods using Hach water quality tests. Samples for chemical analyses were collected at all sites and processed by USEPA compliant Hach methods 10,173 (Total Organic Carbon (TOC)), 8155 (nitrogen as ammonia), 8039 (nitrogen as nitrate), 8153 (nitrogen as nitrite), and 8048 (orthophosphate) within 24 h [Hach Company, Loveland, CO].