Dating Stream Base Flows Using Dissolved Gases and Diurnal Temperature Changes
Abstract
The transit time of groundwater in a watershed between its initial recharge to the water table and its discharge to a stream or river is an important criterion in evaluating the vulnerability the stream to distributed sources of pollution at the land surface. Tracers that have been used most effectively to help estimate transit times, or ages, in young (<50-year-old) groundwater rely mostly on concentrations of dissolved gases (e.g. 3He, SF6, and chlorofluorocarbons (CFCs)). These tracers can be used effectively at wells, but when groundwater discharges to streams the gases quickly reequilibrate with the atmosphere, confounding their use as a dating tool. In order to overcome this challenge, both the stream-flow and gas-exchange parameters must be accurately measured in addition to the tracer concentrations. We present here a simple and cost-effective technique to simultaneously estimate these transport parameters and tracer concentrations. The method uses dissolved gases (argon and nitrogen) and SF6 and/or CFC concentrations in streams to estimate stream-flow and gas-exchange parameters and an effective mean age of base flow. Samples are collected at a single site during one visit to estimate a mean age of base flow upstream from the sampling location. The method uses the fact that diurnal temperature changes in most streams result in a sinusoidal variation in the equilibrium concentration of most gases, with the exact form of the transient concentration signal being a function of three parameters: the mean residence times of the gas and the stream water, and the concentration of the gas in the groundwater inflow. Argon, nitrogen, SF6 and CFC-113 have been measured over 12-14 hour periods at a variety of streams in the mideastern USA. Results suggest the method can be used for small streams after a few precipitation-free days and when the water temperature increases by at least 3 °C during the day. When both SF6 and CFCs are used, a dual-parameter exponential age distribution can sometimes be estimated that is consistent with both tracer results. Further testing is being conducted at additional sites to better understand what other types of gases and range of conditions that might be viable using this method. The method shows promise at being a powerful new tool for estimating groundwater travel times to streams.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.H14D..04S
- Keywords:
-
- 1804 Catchment;
- HYDROLOGYDE: 1813 Eco-hydrology;
- HYDROLOGYDE: 1829 Groundwater hydrology;
- HYDROLOGYDE: 1830 Groundwater/surface water interaction;
- HYDROLOGY