UC Merced

Agriculture in the Central Valley of California is a potential contributor of non-point source pollution to surface waters via the groundwater pathway. This work presents a relatively simple method and inexpensive apparatus for quantifying local groundwater discharge into rivers using heat as a tracer. Two transects along a known gaining reach of the Lower Merced River were used to evaluate the effectiveness of the groundwater discharge monitoring instruments, known as Temperature Javelins, over three months. In terms of fulfilling deployment objectives, Temperature Javelins proved to be low cost, easy to install, and yielded easily interpretable data related to groundwater-surface water discharges. Groundwater discharge velocities were found to vary over time seasonally and spatially (on a scale of meters). Discharges ranged from 0.1 to 6.8 cm/day with higher discharge velocities found on the northern side of the river. These values are consistent with previous values obtained at the same site using other methods. Corresponding hyporheic water samples were collected to investigate solute transport (specifically nitrate) by the discharging water. Nitrate concentrations ranged from nondetectable to 50 ppm in these samples, where elevated values appear to be associated with groundwater entering on the north side of the river. Elevated nitrate flow lines in the hyporheic zone correspond with high discharge areas and alternate with ammonium enriched columns and low groundwater discharge. It is hypothesized that nitrification cells are induced by groundwater-surface water interactions. Testing of this hypothesis is proposed as a subject of further research.