Evaluation Of The Use Of The Sodium Dominance Index As A Potential Measure Of Acid Sensitivity - Final Report

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.

Published: 2006


Pages: 118

Filesize: 2,139KB

Format: pdf

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Executive Summary

In view of the ecological importance of soft-waters for salmonid production, it iscritical that measures are taken to avoid increased rates of acidification, particularlythose relating to changes in land use. This necessitates the identification of acidsensitivewaters. The most commonly adopted indicators are pH (state) and alkalinity(sensitivity). Low pH (<5.5) and low alkalinity (<10 mg L-1 CaCO3) are clearlyindicative of low buffering capacity, but they are extremely variable within any onecatchment, depending on flow conditions and geology. The contribution of sodium(Na+) to the sum of the major cations (Sodium Dominance Index, SDI, or WeatheringIndex) in river waters has been proposed as an indicator of the acid sensitivity of riversof upland Scotland, particularly where sea salt inputs dominate the base cationcomposition. The extent of Sodium Dominance provides a quantitative indication ofcatchment weathering rate, incorporating the effects of diverse geological composition.This project set out to test the following two hypotheses: that SDI is more stable acrossthe range of stream flows than the two most commonly used indicators and is thus abetter indicator of stream sensitivity to acidification than pH and that there is a gradedresponse by the stream macroinvertebrates to values of the index, and hence someecological underpinning of the chemical relationship

These hypotheses were tested by examining the water chemistry of 257 sites acrossIreland, encompassing a range of underlying geologies, during base flow. A furthersub-set of 55 sites were sampled at both base and elevated flow and a number of moredetailed hydrological events were monitored at a smaller number of sites. pH,conductivity, hardness, alkalinity and SDI were determined for each site, together witha range of environmental variables, including geology type, presence or absence offorest, distance from the south-west of Ireland and distance from the sea. The values ofpH recorded for the sites sampled ranged from pH 4.9 to 8.8. Most values were in thecircum-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 significantdifference in SDI was detected between the non-forested and forested sites. Someforested 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 ofsusceptibility of sites to impact. A negative linear relationship between pH and the SDIwas found, for both non-forested and forested sites, individually and combined. Ofparticular interest was the relationship between potentially acid-sensitive sites and theSDI. Although SDI showed some variation, this variation seemed to decline after about5-6 hours of elevated flow, when it again showed less variation than pH or alkalinity. Itappears that a thresh hold level of SDI of 50-60 is indicative of sensitivity toacidification as measured by ANC and alkalinity. A highly significant linearrelationship between SDI and conductivity was found for two groupings of sites (siteswith conductivities > 250µS/cm and 150µS/cm excluded), although there wasconsiderable 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 ofgreater than 60 are at risk of acidity. Acid-Neutralising Capacity (ANC) was calculatedfor 117 sites from the present extensive study. The ANC value fell to zero with anapproximate SDI value of 60. In the same way, when SDI was plotted againstalkalinity, it appeared that an SDI 50-60 represents a critical value in rivers.

Full executive summary in report.