For metals or metalloids in fresh waters (up to 2500 mgL-1 salinity), consider the effect of hardness, pH and alkalinity on toxicity and derive a hardness-modified trigger value (HMTV)a using the appropriate algorithm from table 3.4.3. Table 3.4.4 indicates how the trigger values vary with different ranges of hardness but extra care is needed for waters with hardness below 25 mgL-1 CaCO3. If samples have been filtered, for comparison with the HMTV, this will also take into account suspended organic matter. The hardness algorithms (table 3.4.3) also account for pH.
The recommended decision scheme for metals is illustrated in below ( see Figure 3.4.2) but steps beyond the initial hardness adjustment are optional. If the total metal concentration in the unfiltered sample exceeds the HMTV, then users may choose one or more of four steps:
(i) compare metal concentration with the HMTV after filtering the original unacidified sample through a 0.45 µm membrane filter. An alternative is to proceed directly to measuring filtered concentrations instead of totals initially.
(ii) proceed to more complex estimates of metal bioavailability (step 10) relating to the study site;
(iii) accept that the guideline has been exceeded and institute management action;
(iv) proceed to DTA (step 12).