Ecotoxicology Electrical Conductivity-Magnesium Pulse Framework

The current Water Quality Objectives for Magela Creek have a hierarchy of trigger values and objectives for a suite of analytes based upon both site specific ecotoxicological research and statistical interpretation of existing grab sample data.

Examination of the continuous EC data shows that the fluctuations in EC/Mg detected downstream of the mine typically occur as “pulses” with EC/Mg rising and falling over a matter of hours, often in response to changing hydrologic conditions in the stream, as shown in Figure 1. Clearly there are inherent shortcomings in the ability of weekly grab samples to detect the majority of pulse inputs of mine-derived solutes other than those present at the specific time of sample collection.

As the EC guideline set in the 2004 water quality objectives was derived statistically from grab sample data, is it not appropriate to apply this to short term EC pulse events. Consequently, a new tool is necessary to provide a framework under which continuous EC data can be interpreted, taking account of the wide variation in pulse magnitude and duration.

Grab sample (dots) and continuous monitoring (line) data for Magela Creek from the 2008-09 wet season converted to magnesium concentration.

Figure 1  Grab sample (dots) and continuous monitoring (line) data for Magela Creek from the 2008-09 wet season converted to magnesium concentration.

Based on the findings from a rigorous and extensive program of ecotoxicological test work carried out over the last 4 years an ecotoxicologically derived magnesium (Mg) pulse exposure limit and trigger value framework for Magela and Gulungul Creek has been established.

Pulse exposure development

Electrical Conductivity : Magnesium relationship

Magnesium sulfate (MgSO4) emissions from Ranger Mine are primarily generated from the leaching of waste rock and represent the primary component of the mine site signature in downstream waters. As Ranger mine is the predominant source of solutes in the upper Magela catchment, a strong relationship between EC and Mg concentration has been derived. This relationship allows EC to be used as a reliable surrogate for Mg concentration in creek water downstream of the Ranger Mine. Continuous monitoring of EC, in combination with the strong EC:Mg relationship (y = 0.083, x = 0.47 and r2 = 0.96), effectively provides real-time monitoring of Mg concentrations.

The high reliability linear correlation (Figure 2) between measured EC and the concentration of Mg established using over 6 years of grab sample data enables the concentration of Mg at any time to be inferred from the EC value. Thus EC and Mg are referred to conjointly below.

Best fit relationships between electrical conductivity (EC) and magnesium (Mg) concentrations for the downstream (r2 = 0.96, P<0.0001; black regression line) and upstream 
(r2 = 0.84, P<0.0001; white regression line) monitoring stations in Magela Creek

Figure 2  Best fit relationships between electrical conductivity (EC) and magnesium (Mg) concentrations for the downstream (r2 = 0.96, P

Magnesium pulse exposure testing

To assess the potential toxicity of Mg pulses of various magnitude and duration, a program of pulse exposure tests was conducted which exposed six local freshwater species to a range of Mg concentrations over three exposure durations. A 99% species protection trigger value was calculated for each exposure duration, adding to the existing trigger value for chronic Mg exposures (Table 1).

Table 1 Water quality trigger values for magnesium and electrical conductivity for different exposure durations.
Exposure duration 99% species protection trigger value
Magnesium (mg L-1) Electrical conductivity (µS cm-1)
4 hours 94 1138
8 hours 14 174
24 hours 8 102
Chronic (3–6 days)1 3 42

1Continuous exposure data from van Dam, RA, Hogan AC, McCullough CD, Houston MA, Humphrey CL and Harford AJ. 2010. Aquatic toxicity of magnesium sulphate, and the influence of calcium, in very low ionic concentration water. Environmental Toxicology and Chemistry 29(2), 410–421 [‘van Dam et al (2010)’]

Importantly, it has previously been determined that the presence of calcium (Ca) has a significant impact on ameliorating Mg toxicity. The Mg:Ca (mass) ratio should not exceed 9:1, or Mg toxicity will be increased and the values presented in Table 1 will no longer be applicable (van Dam et al 2010). Full details of the Mg pulse toxicity study will soon be available on the website.

Magnesium pulse exposure limit

The magnesium pulse exposure limit is defined by an exposure duration response curve, based upon testing of six species across four exposure durations with a 99% species protection trigger value calculated for each exposure duration.  The EC pulse exposure limit curve allows for the interpretation of EC pulses occurring over a wide range of magnitudes and durations.  EC pulses of a magnitude that fall below the corresponding pulse duration EC limit value are unlikely to have a biological impact.

The Mg/EC pulse exposure duration limit relationship shown in Figure 3 was produced by fitting a line of best fit to the point data shown in Table 1. This curve enables the interpolation of EC/Mg trigger values for any given pulse duration within the range (from 4 h to continuous exposures of at least 72 h) of durations that have been tested.

Table 2 lists the ecotoxicologically derived pulse exposure limits for various exposure durations. Exposure durations of less than 4 h will be assigned a limit of 1140 µS cm-1. The chronic exposure limit, for exposures exceeding 72 hours, is 42 µS cm-1 or 3 mg L-1 Mg. The highlighted values in Table 2 are values measured by ecotoxicological testing and all other values are interpolated using the inverse third order polynomial function fitted to the curve in Figure 3.

Table 2 Ecotoxicologically derived EC and Mg trigger values (99% ecosystem protection level) for short duration pulse exposure
Pulse duration (h) Trigger value
  EC (µS/cm) Mg (mg/L)
1138 94
4 1138 94
6 300 24
8 174 14
10 146 12
12 136 11
14 129 10
16 123 9.8
18 118 9.3
20 112 8.9
22 107 8.4
24 102 8.0
30 89 6.9
36 78 6.0
48 62 4.6
60 50 3.7
72 42 3.0
Magnesium pulse exposure limit curve, expressed as EC.

Figure 3 Magnesium pulse exposure limit curve, expressed as EC.

Continuous data interpretation framework

Definition of an Electrical Conductivity (Mg) pulse

An EC pulse is defined as any increase in EC at the downstream monitoring site above the 42 µS cm-1 chronic exposure limit. The duration and magnitude of a pulse will be defined by the approached summarised in Figure 4. In this context it should be noted that the pulse width is defined by the time the magnitude of the pulse is above the chronic (most conservative) toxicity criterion. Furthermore it is additionally conservatively assumed that the peak value applies across the full duration of the pulse. The duration and magnitude are then compared to the appropriate exposure limit in Table 2. Pulses whose magnitude and duration characteristics fall below the curve will satisfy the 99% species protection level and it can be concluded that a biological impact is unlikely for these cases.

Occurrences in which two or more pulses occur within a short time period (≤ 72 h) will be assessed by SSD on a case-by-case basis and may be considered as a single pulse if appropriate.

Magnesium pulse exposure limit curve, expressed as EC.

Figure 4 Magnitude and duration of an Electrical Conductivity pulse.

SSD will interpret the continuous monitoring EC data for 2012-13 wet season using this ecotoxicologically-derived EC pulse framework. Reporting of data will be posted to the following web pages on a weekly basis (7 days in arrears) during the wet season, changing to monthly during established recessional flow conditions during the dry season.