Final Report (2000-LS-3.2.1a-M2) - Cruikshanks et al
Summary: Evaluates the efficacy of the SDI approach to site designation under Irish conditions. The water chemistry of 257 sites across Ireland were examined during base flow. A further 65 sites were sampled at base and elevated flow, and more detailed hydrological events were monitored at a smaller number of sites.
In view of the ecological importance of soft-waters for salmonid production, it is critical that measures are taken to avoid increased rates of acidification, particularly those relating to changes in land use. This necessitates the identification of acid sensitive waters. The most commonly adopted indicators are pH (state) and alkalinity(sensitivity). Low pH (<5.5) and low alkalinity (<10 mg L-1 CaCO3) are clearly indicative of low buffering capacity, but they are extremely variable within any one catchment, depending on flow conditions and geology. The contribution of sodium(Na+) to the sum of the major cations (Sodium Dominance Index, SDI, or Weathering Index) in river waters has been proposed as an indicator of the acid sensitivity of rivers of upland Scotland, particularly where sea salt inputs dominate the base cation composition. The extent of Sodium Dominance provides a quantitative indication of catchment weathering rate, incorporating the effects of diverse geological composition. This project set out to test the following two hypotheses: that SDI is more stable across the range of stream flows than the two most commonly used indicators and is thus a better indicator of stream sensitivity to acidification than pH and that there is a graded response by the stream macroinvertebrates to values of the index, and hence some ecological underpinning of the chemical relationship
These hypotheses were tested by examining the water chemistry of 257 sites across Ireland, encompassing a range of underlying geologies, during base flow. A further sub-set of 55 sites were sampled at both base and elevated flow and a number of more detailed hydrological events were monitored at a smaller number of sites. pH, conductivity, hardness, alkalinity and SDI were determined for each site, together with a range of environmental variables, including geology type, presence or absence of forest, distance from the south-west of Ireland and distance from the sea. The values of pH recorded for the sites sampled ranged from pH 4.9 to 8.8. Most values were in the circum-neutral range (pH 6.5-7.5). SDI values for all sites sampled ranged from 10.1 to81.9: the highest values of SDI were recorded in upland sites in Wicklow, Donegal, Galway and Kerry. Within any geologically classified group of sites, no significant difference in SDI was detected between the non-forested and forested sites. Some forested sites become substantially more acidic than similar non-forested sites, and yet the two had similar SDI values, thus the index is not an impact, but an indication of susceptibility of sites to impact. A negative linear relationship between pH and the SDI was found, for both non-forested and forested sites, individually and combined. Of particular interest was the relationship between potentially acid-sensitive sites and the SDI. Although SDI showed some variation, this variation seemed to decline after about 5-6 hours of elevated flow, when it again showed less variation than pH or alkalinity. It appears that a thresh hold level of SDI of 50-60 is indicative of sensitivity to acidification as measured by ANC and alkalinity. A highly significant linear relationship between SDI and conductivity was found for two groupings of sites (sites with conductivities > 250µS/cm and 150µS/cm excluded), although there was considerable scatter in the data. The data suggested that an SDI value of between 40-60identified sites which are at potential risk of acidification, while sites with values of greater than 60 are at risk of acidity. Acid-Neutralising Capacity (ANC) was calculated for 117 sites from the present extensive study. The ANC value fell to zero with an approximate SDI value of 60. In the same way, when SDI was plotted against alkalinity, it appeared that an SDI 50-60 represents a critical value in rivers.
Full executive summary in report.