Nitrate fluctuations in groundwater

Many time-series data of nitrate concentrations in groundwater show within-year fluctuations. These fluctuations are different in different locations.

Where suitable continuous groundwater level records are available nearby, a more or less close relationship between groundwater levels and nitrate concentrations can often be observed. Higher nitrate concentrations are typically associated with higher groundwater levels.

This project aimed to identify what links these within-year concentration fluctuations to changes in water levels.

Within-year nitrate fluctuation mechanisms

Four conceptual models were prepared to represent scenarios where groundwater recharge is taking place in chalk by only one mechanism.

These mechanisms were:

• winter piston flow through the unsaturated zone matrix
• winter bypass flow from the base of the soil, bringing high-nitrate water directly to the water table
• water table rise, from water entering elsewhere in the catchment flushing out pore water
• change in flow path, giving access to a greater percentage of shallow high nitrate water

The research evaluated these mechanisms in a simple way to determine the relationship between nitrate concentration at the water table and water level. We concluded that all mechanisms except flushing by a rising water table would be likely to result in the more-or-less simultaneous rising of the water level and the arrival of nitrate.

Field measurements to test the conceptual models

We used a hanging sampler to test some of these mechanisms at a site on the Chalk aquifer in Hampshire. This comprised a series of small plastic bottles, each containing a plastic ball that sealed the bottle as it filled. The sampler was hung above the water table during a low water-level period, so that the bottles filled successively as the water level rose. The bottles were spaced a few centimetres apart.

The interim results highlighted a number of features. For example, at 20 m below ground, different quality water is observed entering the borehole.

Further reading

Contact

Please contact Dan Lapworth for further information.