Thermal dynamics of a riparian aquifer subject to flooding : lower Colorado River, Texas, USA

Abstract

The zone of mixing between rivers and the riparian aquifers adjacent to them, known as the hyporheic zone (HZ), is critical for water quality and water resource issues. Hyporheic mixing within the sediment provides a unique nexus of nutrients and environmental factors that promotes important biochemical reactions such as respiration, denitrification and anaerobic ammonia oxidation. Biochemical reaction rates are directly related to temperature. Moreover, temperature is a useful environmental tracer that may be used to infer groundwater flow paths in the HZ. In this study, I investigate coupled fluid flow and heat transport dynamics in riparian aquifers adjacent to the river channel during three flood events. I present 2D riparian aquifer temperature data from two sites along the Lower Colorado River (LCR). These data, along with river and groundwater table elevation data, allowed observation of riparian aquifer temperature responses to the flood pulses as they traveled downstream. At the downstream site (Webberville Park), pre-flood temperature penetration distance into the bank suggested advective heat transport from lateral exchange of river water into the riparian aquifer was occurring during relatively steady river flow conditions. While a small (20 cm) dam-controlled flood pulse had no observable influence on groundwater temperature regimes, larger floods (40 cm and >3 m) caused lateral movement of distinct heat plumes away from the river during flood stage, and retreated back toward the river after flood recession. We interpret these plumes as heat transport by advection caused by flood waters being forced into the adjacent riparian aquifer. These flood-induced temperature responses were controlled by the size of the flood, river water temperature during the flood, and local factors at the study sites, such as topography and local hydraulic gradients. For the two larger events, the thermal disturbance lasted days after flood waters receded, suggesting that large floods have long lasting impacts on the temperature regime of riparian aquifers beyond the time scale of the flood itself. These persistent flood-induced thermal disturbances likely have a significant impact on biochemical reaction rates and nutrient cycling in the river system.