Water Chemistry in the Confluence Zone Downstream a Limestone Treated Lake and an Acid Tributary: Principal Component Analyses Including Warm and Cold Winters and an Episode High in Sea-Salts

Abstract

Extensive limestone treatment of lakes and watercourses has been carried out especially in Norway and Sweden to counteract effects of acidification. Lakes have been the most commonly treated part of the water systems. However, treatment of lakes upstream acid tributaries may introduce downstream toxic mixing zones for fish. To sort this out the outlet of a treated lake, a downstream acid tributary and two sites in the confluence zone were intensely monitored during a period of 28 months. The data accumulated come from a period (from February 1992) where significant climatic variations took place that provide a basis for studying intercorrelations between water chemistry and climatic change. The first two winters were warmer than normal and the catchments were hardly covered with snow and the lake was ice-covered only for a few days. The last winter was colder than normal, nival and the lake was ice-covered from December to April. During the second winter a low pressure over the north Atlantic gave strong south-westerly winds and large amounts of precipitation loaded with sea-salts. The principal component analysis (PCA) separates both the outlet and the tributary data into two groups while the data from the confluence zone are separated into three. Significant different water chemistry was observed in the outlet during the ice-covered period while effects of the sea-salt event splits the data from the tributary into two groups. The water chemistry in the confluence zone reflects both the ice-covered period, the sea-salt event and besides the more general situation. The PCA analysis indicates that the changing weather conditions mainly influenced on the water quality in the tributary. The water quality in the confluence zone was generally a conservative mixture of the outlet and the tributary waters except for alkalinity, H+ and inorganic aluminium (Ali). Generally, hydrolysing Ali in the confluence rendered the water quality highly toxic to fish. The potential toxicity increased during the sea-salt event and during the ice-covered period of the lake due to increased concentrations of Ali both in the outlet and the tributary waters. The results indicate that this may be a general problem in confluence zones downstream limestone treated lakes and acid tributaries.