3.4 Contaminant loading to the receiving environment

A key quantity required in the assessment of potential impacts from water infiltrating through mine wastes on surface water quality is an estimate of the solute load when groundwater discharges into a stream or other water body. The solute load L (mg/d) is the mass of solute that enters the water body over a specified time period. The mass load is calculated by multiplying solute concentration by the volumetric groundwater discharge rate, as shown in Equation 1.

where:

Estimates of current loads can be calculated with field measurements, while future loads are often predicted on the basis of a groundwater flow model and a solute transport model. Given an estimate of the solute load delivered to the stream bed, and a mixing model for solutes once they enter the stream, it is possible to estimate the concentration of the contaminant of concern in the stream as it exits a defined mixing zone. This estimate can then be compared, for example, to the aquatic water standard on solute concentration or the water quality standard for use of surface water as a water supply for irrigation.

Figure 14 illustrates this concept, where a set of hydraulic head measurements in piezometers installed in the valley bottom beyond the toe of a waste rock dump have been contoured to provide input to a Darcy law calculation of distributed groundwater discharge along the stream course. The waste rock pile was releasing acidic drainage due to sulfide mineral oxidation. Concentration data from monitoring wells have been contoured to provide an estimate of a dissolved zinc plume at the stream bank. The loading calculation requires an estimate of the hydraulic conductivity of the valley-fill sand and gravels. Values calculated from response tests in individual piezometers, and constant rate pumping tests, ranged over several orders of magnitude. For the purpose of this loading calculation a value of 6 x 10^-4 m/s was chosen. Using these data, the estimated zinc loading to the stream in September was 80 kg/d, while in November the loading increased to an estimated 400 kg/d. These data demonstrate significant temporal variability in plume geometry and solute loads in this high-permeability system. The data for September reflects conditions at the end of the dry season; the data for November reflects conditions after the onset of winter rains and first flushing of the stockpile since the end of the previous wet season. It is important to acknowledge the uncertainty associated with these estimates arising from the uncertainties in defining the groundwater flow volumes using a contour map of water table elevation, the plume maps derived from a limited number of observation wells, and the challenge in estimating a representative value for the hydraulic conductivity of the sand and gravel deposit.

Figure 14 – Example illustrating the estimation of zinc loading from the groundwater system to a creek as it passes the toe of a waste rock facility yielding acidic drainage to the sand and gravel unit.

Contaminant loads to streams are sometimes estimated using synoptic stream sampling (e.g., Byrne et al., 2017). In this approach, estimates of groundwater flow volumes entering the stream are computed by making measurements of stream water velocities and cross-sectional flow area at close intervals along the stream course. At the same time a number of water samples are collected along the stream to record contaminant concentrations. Inflows of surface water along the stream segment being analyzed must also be measured to complete the mass balance calculations to estimate solute loads from contaminant sources distributed along a stream.