A comparison of methods for estimating groundwater-surface water interactions in braided rivers

Abstract

Understanding how groundwater and surface water bodies interact is an important component of freshwater management. The direction and quantity of the flow between these two systems can vary in time and space, and these processes play various hydrological and ecological roles. The exchange of groundwater and surface water impacts water quantity, nutrient cycling, contaminant transport, and temperature regulation in surface water bodies for aquatic organisms. The interactions between these two systems can be difficult to measure and is often a poorly understood component of water budgets. Characterising these exchanges in gravel-bed braided rivers and their surrounding aquifers can be more difficult than in other environments due to their highly heterogeneous substrate; very permeable streambeds and subsurface material; dynamic geomorphology and flow levels; and difficulty installing direct measurement equipment into the coarse-gravel riverbeds.

In this study, mini-piezometers and vertical temperature probes were installed, and physicochemical analysis was carried out on the Hakatere/Ashburton River on the South Island of New Zealand. The methods were used to identify the direction of flow between groundwater and the river and quantify the rate of seepage through the streambed. Results from the methods were compared to assess their effectiveness for use in a braided river system. From a practical perspective, the purpose-built mini-piezometers and vertical temperature probes proved effective in this dynamic coarse-gravel environment. Results across the methods provided a complex picture of groundwater-surface water processes at the study sites, revealing areas of upwelling and downwelling through the streambed. The results reinforce the benefits of multi-method studies for investigating exchanges in groundwater and surface water, as they may better capture temporal and spatial variations in these flows, while providing robust study designs that allow for comparison of results across methods and redundancy in case of equipment or data collection failure.