EVALUATING E. COLI, DISSOLVED ORGANIC CARBON, AND TOTAL DISSOLVED NITROGEN WITHIN TWO SOUTHEASTERN PUERTO RICO WATERSHEDS

Surface water is a critical resource both for human consumption and for agriculture. Human impacts can add biologic material (including bacteria) to streams and rivers and increase the concentration of nutrients including nitrogen. Such changes can affect oxygen saturation through aerobic decomposition of organic material.

We sampled river water from 15 sites in two adjacent southeastern Puerto Rico watersheds, the Río Guayanés and Río Maunabo drainage basins (68.4 km²; mean annual flow, 2.14 cms and 13.9 km²; mean annual flow, 0.56 cms, respectively). We measured dissolved oxygen using a YSI meter and E. coli using Aquagenex bag tests incubated in the field. We measured dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) at the University of Vermont using as Shimadzu carbon analyzer. Water samples were collected in both March and June 2022; dissolved oxygen and E. coli were measured only in June. Stream flow data were downloaded from the USGS water portal.

Dissolved organic carbon concentrations varied across sites and sampling times from 0.9 to 14.9 mg/L (average 2.68 mg/L) but were generally higher in March. TDN varied from 0.15 to 1.18 mg/L (average 0.38 mg/L) and C/N ratios varied from 1.7 to 61.9, averaging 9.2. Dissolved oxygen saturation ranged from 86 to 122 percent with an average of 97%. River flows were higher in March than in June due at least in part to 3-day February storm that dropped more than 10 cm of rain on the region. E. Coli was present in all samples.

These data allow us to see a snapshot in time on these rivers. The 11 highest TOC concentrations were measured in river water samples collected in March. Thus, high stream flow appears to increase total organic carbon concentration in the water as soil-derived organic matter moves into streams. Eleven of fifteen samples analyzed for E. coli in June had MPN/100 ml values over 100 and two samples had > 1000 MPN/100 ml. These data indicate that the water at most sites we sampled would have been unsafe to drink at that time. We use these data, together with other water quality parameters, to evaluate the dependence of river water chemistry on different land uses and hydrologic conditions and to guide the process of hypotheses generation related to landscape-scale influences on river water quality.