WaterSubject

The construction of Upper Tamar Lake (UTL) in the South West of England, completed in 1975 was undertaken to replace water supplies from the smaller Lower Tamar Lake (LTL) and to provide drinking water to North Cornwall and North West Devon. Built ‘on-line’ in the headwaters of the Tamar catchment, the area of the reservoir is comparatively small, being 32.2 ha, with a corresponding drainage catchment area of 1320 ha. The reservoir has a maximum depth of 12.5 m and a gross storage of 1364 ML (net storage of 1335 ML). Being small relative to the catchment area upstream, the reservoir recharge is rapid and it is guaranteed to spill each year irrespective of the severity of drawdown. A compensation flow of 2.76 ML/day (0.03194 cumecs) is released at all times into the River Tamar via LTL. LTL has a surface area of 14.1 ha and a capacity of 235 ML. Since 1975, siltation and encroachment by marginal vegetation have reduced these dimensions. A recent bathymetric survey recorded a maximum depth of 4.44 m. LTL is fed by the UTL subcatchment and several small tributaries from a catchment of 494 ha.

Land use within the Tamar Lakes subcatchment comprises predominantly intensive livestock agricultural units (14 farms), particularly dairy and beef. Since its construction in 1975, UTL is widely considered to have been subjected to continual agricultural nutrient enrichment. The catchment contains no known significant nonagricultural nutrient sources, such as point source discharges from sewage treatment works.

From the outset, algal succession and maturation were rapid in UTL and synonymous with a eutrophic/hypereutrophic condition; blue-green algae were common. After 22 years of operation, concerns were expressed over algal and water quality problems within the reservoir, together with the perceived impacts that these were having on the River Tamar itself, in particular its salmon and trout stocks. As a result, an investigation was commissioned by South West Water (SWW) Ltd. with a view to understanding the causative factors, identifying the primary drivers and developing solutions to the perceived problems.

Numerous solutions were assessed in anticipation of the environmental requirements that are expected in the Water Framework Directive (WFD), together with the role of the Common Agricultural Policy (CAP) in influencing land management practices and water quality. The relationship between these wide-ranging policies and legislation was explored with respect to environmental damage, financial liabilities and responsibilities, and ultimately the pursuance of truly sustainable economic, agricultural and environmental practices.

The investigation comprised analysis of several complementary routine monitoring programmes (1970–2002) together with data obtained from continuous water quality sampling and loggers (2001 and 2002). The first survey programme was the routine river monitoring (RRM) methodology commonly used by the Environment Agency and its predecessors, whereby spot samples were taken at regular intervals throughout the year. Data from this ongoing programme were used, but confined to the period 1970–2002. Secondly, continuous river monitoring (CRM) data were collected by SWW using in situ multiparameter sondes with integral data-loggers. Sampling occurred every 15 min at three sites: between, upstream and downstream of the Tamar Lakes. Continuous data were collected for turbidity, pH, dissolved oxygen, conductivity and ammonium covering the period from 5 November 2001 to 11 September 2002. Calibration was undertaken on three occasions during each week of deployment. Finally, in conjunction with the CRM, more detailed data were collected, triggered by rainfall events and under spate conditions (continuous river spate monitoring, CRSM). Automated water samplers were used to collect samples every 2–4 h during spate events upstream and downstream of the Tamar Lakes. Analyses were undertaken at SWW (UKAS accredited) laboratories for suspended solids (SS), total phosphorus (TP), total reactive phosphorus (TRP), nitrate and ammonium.

In contrast with other evaluations of the water quality problems in the UTL subcatchment, this investigation included all outlying data values from the RRM programme. Previous reports adopted a practice of removing ‘fliers’ using subjective methods. However, a more ‘open’ approach of all data inclusion was applied, thereby enabling an assessment of high river flows and spate events. Such events are regarded as important in influencing water quality in the Tamar system and ultimately nutrient inputs to UTL. As an illustration of the changes in nutrient flux within the system over the period 1970–2002, It presents data (RRM) for nitrate (mg/L N) in the River Tamar immediately upstream UTL, within UTL and downstream LTL.

Nitrate concentrations have clearly increased over time in the River Tamar upstream of UTL, contributing to ever-increasing nutrient loads within the reservoir. The increase in nitrate concentrations within the River Tamar upstream of the reservoir is equivalent to a rate of 0.08 mg/L N/year. This increase mirrors very closely the pattern of increasing nitrate fertiliser use during the 1970s and 1980s that occurred as a consequence of agricultural intensification. A similar pattern was evident for total hardness above the lake, potash being administered to land in the form of potassium carbonate as part of the general farming regime.

However, it was also apparent that a substantial and statistically significant (P<0.01) reduction in nitrate is evident as water passes through the reservoir and is discharged downstream as either compensation or spill water. The observed reduction in nitrate is likely to be due to biological processes within the lake, notably algal primary productivity that utilises the nutrients as the biomass develops. Similar results were found for phosphate and silica concentrations. In addition to biological activity, a certain proportion of N and P reductions will be due to SS reduction and deposition in UTL.