Water Chemistry of the Weaver

Freshwater is so-called because of the low amount of dissolved solids that it contains. As dissolved solids increase, the salinity of the water can be said to be higher and as the salinity increases there will be an effect on the aquatic biota. The river Weaver is relatively unusual in that it receives inputs of enough dissolved salts from various parts of the brine extracting and chemical industries to significantly affect freshwater organisms.

Conductivity can be used as an overall measure of the total amount of dissolved solids in the water and is a particularly useful way of studying small-scale changes of water quality in the field. There is always background conductivity in freshwaters due principally to low levels of dissolved ions and partly due to the water itself. Unfortunately, a conductivity meter does not distinguish between the different ions contributing to the overall value so that it is not possible to determine how, or in what form salts have been added.

There are two principal sources of extra dissolved salts entering the River Weaver in the study area - NaCl principally from the Holford brinefields and CaCl₂ from the waste discharges from factories involved in the manufacturing of soda ash. Inputs can be direct or via settling beds and lagoons. The two main operating sites (both owned by ICI) are currently the Lostock and Winnington works although the Wallerscote works was closed only a few years ago.

The conductivity of the lower river is very variable, partly because industrial processes are likely to vary greatly in their output of effluent from time to time and partly because of major alterations in water flow that are caused by weather. Judging from a number of small, unpolluted watercourses entering the main river, the background conductivity for the area seems to be in the region of 600 µs cm^-2 (cf upper Mersey - 544 µS cm^-1, Holland & Harding, 1981). The main rivers above Northwich (the Dane and the Weaver) have conductivities elevated above the natural background ( 1,500 - 2,000 µs cm-2 in the R Dane at the A533 Bridge and 2,000 µs cm^-1  in the R Weaver at Hartford Bridge, on 24/10/90) as they enter the study area, presumably due to the dispersed effects of effluents from the brinefield. The combined water from two small tributary brooks (Wade and Wincham Brook), one of which receives effluent from the Lostock site, is responsible for a significant raising of the conductivity in the Weaver (to 4-5000 µS cm^-1 on 24/10/90), although at the confluence, the river still supports a good fishery based on roach and bream (various Northwich AA members, pers comm.). This is despite the possibility that there may be a significant negative impacts on susceptible species at these levels of pollution.

Extreme conductivity values of >30,000 µS cm^-1 that can occasionally be detected in some watercourses are derived from direct discharges to the river of the so-called “DBO liquors” from the alkali factories. These are very dense and can form layers of very ‘salty’ water on the bottom of the river.

By the time that the water reaches Acton Bridge on the main course of the Weaver, the high conductivity discharges have been well mixed with the remainder of the water column and as a consequence the maximum levels of conductivity observed at this point are > 16,000 µS cm^-1.

Both calcium and sodium chlorides are not very toxic but are involved in influencing the osmotic balance of aquatic organisms. Excessive levels of chlorides will cause problems for freshwater organisms. The 96hr LC50 for goldfish was 7341 mg l^-1 (Adelman et al 1976). In a study of the effects of road deicing salt on streams, Crowther and Hynes (1977) noted chloride levels up to 1770 mg l^-1 in urban streams. Laboratory tests on the drift of invertebrates showed that a detectable increase occurred around 1000 mg l^-1. Shaw et al (1975) found the growth of Atlantic salmon parr to be similar at chloride levels at 0.1 - 20 % although 20% was iso-osmotic with parr plasma. Thus, it can be seen from the results contained in the various studies on the Weaver, that chloride concentrations can reach problematic levels for freshwater organisms, even in the absence of more toxic materials.

Ammonia is a natural component of the nutrients present in rivers although elevated concentrations are usually only typical where there are high levels of organic pollution. It is readily soluble in water forming an equilibrium solution between the ionised and unionised forms. The toxicity of the solution depends on the amount of unionised ammonia and this is related to temperature, salinity and pH (see Wheaton 1977). Large amounts of Ammonia enter the river Weaver from the effluents from the chemical works where it is used as a catalyst in the Solvay process, even though there has been considerable improvement over the years. Low dissolved oxygen concentrations can also exacerbate the impact of ammonia on aquatic life.