Restoring Acid Waters: Loch Fleet 1984-1990

The underlying principles of limnology, the fruit of much scientific endeavour in the latter part of the nineteenth century and in the early decades of the twentieth, reach beyond the purely scientific or academic pursuits to the development of management strategies to maintain or improve water quality of acid surface waters and their dependent biological communities. The more recent advances in understanding acidified waters have been applied similarly to practical methods of alleviating their unwelcome effects. This book reviews a number of these techniques and their applications and, through the experience of the individual contributors to the Loch Fleet project, 1984 to 1989, considers the results gained there in the wider context of activities with similar objectives elsewhere.

To put the Loch Fleet project into scientific perspective, some theoretical background is needed. Loch Fleet is a recently acidified system. It once sustained a small trout population which had died out by the 1970s, and it is known that water quality at that time was not suitable for trout (see Chapter 3). Diatom stratigraphy has also demonstrated that the loch has become more acid in recent years (see Chapter 14). What processes caused it to become more acid? How has acidification there been related to acid deposition and change in land use? Why is catchment manipulation necessary — will the lake not simply recover as acid deposition declines under the influence of present emission control legislation? This chapter sets out to answer these questions and to give a basic understanding of acidification and recovery mechanisms.

This chapter provides information about the fishery status at Loch Fleet prior to liming and on the preliminary laboratory and field bioassay tests from which a target water quality was derived. Some comparisons with other upland waters in the UK are also included.

In this book it is our aim to review a variety of practical methods and procedures aimed at restoring acid waters, and to assess the liming project at Loch Fleet in the light of other selected examples. Some of these examples have already been identified in Chapter 1 (Table 1.1) and other examples, where there is relevant information, will be used throughout this text to develop the principles and practice of restoration. In reality, most of the methods that have been adopted have been based on applications of “lime” (as calcium oxide, calcium hydroxide, or calcium carbonate), or sometimes other materials such as magnesian limestones or calcium silicate slag. Some mention will be made here of the more limited experience reported for other materials and methods.

In any catchment acidity mitigation programme it is essential to understand and quantify the hydrological and biogeochemical processes of the catchment. Quantitative and qualitative measurements of deposition or atmospheric inputs to a catchment are thus important. Catchment hydrological and hydrochemical models (see Chapter 6) require deposition input data and, in association with a complementary programme of stream water or runoff measurement and ionic analysis, measurement of deposition inputs provides data for the calculation of hydrochemical input/output budgets or fluxes. The importance of these are explained and demonstrated in Chapters 12 and 16.

Hydrology is important in acidification and liming studies since it is evident from many investigations that changes (both short and long term) in surface water quality are related to stream flow or lake turnover. This reflects the pathways by which rain reaches surface waters, contributions of groundwater and intercepted fog and mist, and the dynamics and kinds of chemical and biological reactions that occur during this transfer. Since groundwater usually has a long residence time within underlying mineral materials in a catchment, it has higher conductivity and alkalinity. Thus, when atmospheric input is low (drought) the hydrological base flow reflects groundwater composition, rather than rain composition. Conversely, following heavy rains or snow melt, the base flow contributes little to the total, and acidity deposited from the atmosphere, or mobilised during heavy runoff from upper soil layers, will predominate.

A sound description of the flow pathways and residence times of water is widely recognised to be a prerequisite for understanding and modelling the mechanisms of surface water acidification (Hornung et al., 1986). Similarly, a quantitative description of how catchment liming will reverse acidification, in particular the duration and intensity of reversal in different situations, presupposes an adequate knowledge of catchment hydrology.

An understanding of the chemistry of Loch Fleet and its drainage waters necessitates a study of the chemical interactions between rainfall, vegetation, and soil. The vegetation can exert a strong influence on bulk deposition inputs through physical interception and chemical reactions at leaf surfaces. Further modifications resulting from interaction with soil depend on the hydrological pathways taken from soil to stream and lake.

At Loch Fleet the liming strategy was based on both laboratory and field trials of lime applications to moorland soils, the design of the latter being based on the results of the former. Innovative treatments, for example moorland burning and forest fertilisation, were also attempted, and have been compared with the performance of the liming treatments.

Direct liming of acid waters to prevent fish mortalities has been practised for several decades in hatcheries and other waters (Jensen and Snekvik, 1972) and is more commonplace than catchment liming. The scientific basis of direct water liming is well established in the literature, whilst relatively little scientific investigation of catchment liming had been reported prior to the start of the Loch Fleet project in 1984. For this reason, a major objective of the Loch Fleet project was to advance the science of catchment liming and only where alternative forms of treatment could complement catchment liming were they considered as part of an integrated strategy. In particular, the possible role of direct lime dosing or in-stream treatment targeted to spawning streams to provide a higher degree of protection for the early life-stages of salmonids was investigated.

Soils modify the composition of incoming precipitation passing through them to streams or lakes by interaction between the dissolved constituents and ions adsorbed on the clays and organic matter that make up the soil exchange complex. Hydrogen and aluminium are released when those ions adsorbed on the exchange complex greatly exceed adsorbed nutrient bases. Conversely, high levels of adsorbed bases buffer the soil against acidity in incoming precipitation. Basic ions on the exchange complex can be replenished from easily weatherable soil minerals and afford long-term buffering against acidification.

A target water quality for fish and other aquatic fauna of a liming project is discussed in Chapter 3. In brief, the pH and calcium concentration need to be raised and the toxic inorganic fraction of total aluminium reduced. As mentioned in Chapter 3, the specific target water quality of a liming project will be dependent on a number of factors. At Loch Fleet. since the objective was to improve the water quality so that brown trout survival and reproduction was possible, the target water quality was set at pH greater than 6, calcium concentration greater than 100 µeg litre ¹, and monomeric inorganic labile aluminium less than 30 µg litre ¹. A regular programme of sampling of surface waters started early in the project to assess the extent to which the water quality would need to be improved and, since liming, to establish the water quality changes that followed. Water quality data for pH calcium, and aluminium are obviously crucial to judging the success of an acid mitigation project. Sampling and analysis of surface waters, of course, can extend further, and in many instances a more comprehensive programme is implemented to accompany a programme of analysis for deposition inputs (see Chapter 5). This allows hydrochemical input/output budgets or flux calculations to be made. These can often help to understand acidification processes in a catchment and provide a way of predicting the duration of treatment effectiveness (see Chapter 16).

An earlier section of this book (see Chapter 1 and Chapter 3) provides a review of the loss of fisheries attributable to acidification of their freshwater habitat, both in general and at Loch Fleet. This chapter will review the progress of fishery restoration at Loch Fleet subsequent to the liming applications in 1986 and 1987. This programme has involved confirmation of the water quality targets, the reintroduction of stock to the loch, and surveillance of the successful spawning and fry development in the improved water conditions.

While the response of fish reintroduced to the Loch after liming and other treatments was a primary objective of the Loch Fleet study, the response of other aquatic ecosystem components was clearly important. Studies commissioned included analysis of the diatom flora (both past and present), microbiology, primary production, zooplankton communities, benthic and littoral fauna and flora, and specifically the beetle communities, potentially competing with the introduced fish as predators. For most of these studies the limited pre-liming period allowed only some sampling opportunities in the initially acid conditions, while post-liming studies have continued for more than 4 years after lime treatments.

Although the focus of many restoration/liming studies lies with the improvement of surface water quality and its associated fauna and flora. a variety of studies on the associated terrestrial ecosystems have been undertaken at Loch Fleet, as well as at other restoration sites. The objectives are partly to deduce soil and hydrological conditions as revealed by vegetation, and to assess the contribution of terrestrial components, for example for fish diets, and furthermore to provide a basis for assessing any changes in the associated terrestrial environment as a consequence of liming or other restoration techniques.

In this chapter the targets for acceptable drainage water quality of liming treatments will be explored, and the ways in which predictions can be made of the length of time over which liming treatments may be expected to produce drainage water quality above the established target value will be discussed.

It has been the aim, in this book, to deal with the principles and practice of catchment liming based on what has been learned from the Loch Fleet project and by reference to experience reported for other liming studies. Source material for the preceding chapters is derived from diverse disciplinary fields. This chapter aims to integrate and assess the results of all these areas of investigation and to provide an appraisal of the Loch Fleet project as a catchment liming experiment, placing what has been learnt at Loch Fleet in the context of other projects aimed at mitigating acidification.