Recent research in upland areas is showing that groundwater is far more important in the overall hydrological system than was previously thought. Groundwater discharging to streams is critical in maintaining stream flows (especially at low flow times), influencing stream water chemistry, moderating temperature extremes, and maintaining environmental conditions to support stream biodiversity. But we still understand little of how groundwater occurs and behaves in upland areas. Work at Talla is providing more evidence to help improve understanding.
Five shallow boreholes (< 2–4 m) at Talla are helping to show where and how groundwater is flowing. As well as using the boreholes to test how fast groundwater is flowing through the ground, they are also being used to investigate how groundwater levels change throughout the year. In two of the boreholes we have installed automatic monitors — nicknamed Divers. These have been measuring groundwater level and temperature every 15 minutes since May 2008.
|Groundwater data calendar|
May 2008 – July 2009
once per month monitoring
May 2008 - May 2010
once per six month monitoring
Borehole 1 is drilled into permeable alluvial gravels, only metres from a large stream. Unlike groundwater, streams react quickly to changes in rainfall and air temperature. The water level and temperature data from Borehole 1 show how groundwater here is strongly influenced by the nearby stream. The groundwater level in the borehole can rise by nearly 40 cm overnight after heavy rainfall. Unlike most groundwater in the UK, which tends to stay at the same temperature year-round, the temperature of groundwater in this borehole varies by more than 7°C from summer to winter. This is because the nearby stream water doesn’t just flow over the ground surface, but infiltrates into the gravel that forms the stream bed — in other words, it quickly recharges the shallow gravel aquifer to form groundwater.
Borehole 3 is drilled higher up, away from the stream, into lower permeability glacial till and moraine deposits. It shows a very different pattern of groundwater level and temperature fluctuations. Groundwater level rises and falls more slowly, typically taking at least one month to rise by 40 cm, and falling even more slowly. However, the difference between the lowest and highest groundwater levels over the year is similar to Borehole 1, at about 0.5 m. Groundwater temperature, however, varies by less than 1°C over the year, showing that the groundwater here is more insulated from changes in air temperature. Recharge to the shallow aquifer here, and groundwater flow through the aquifer, are slower than in the alluvial gravels at Borehole 1.