Oscar Benavente, Consultant. Geochemistry and mine closure, SRK Consulting Chile
Luis Guerrero, Junior Consultant. Geochemistry and mine closure, SRK Consulting Chile
The use of satellite images allows the visualization of the Earth’s surface, including estimation of physical and chemical parameters of surficial water bodies. As reported in some crater lakes, the colour of different water bodies can be related with the physico-chemical characteristic of the water (Ohsawa et al., 2009; Murphy et al., 2017): (i) the blue component results from Rayleigh scattering of sunlight by very fine aqueous colloidal sulfur particles, (ii) the green component is attributable to absorption of sunlight by dissolved ferrous ions, and (iii) the red and yellow colors result from the presence of Fe-hydroxides colloids.
We have applied this methodology to water bodies affected by mine drainage, specifically to Au and Cu pit lakes and Cu tailing ponds having distinctive pH values (between 3.5 to 9.5; Figure 1). We have accessed to different Landsat images (4, 5, 7 and 8) over the study period (Jan 1st, 1995 to Jan 1st, 2019) using the Google Earth Engine. After filtering the cloud-covered images, the blue, green and red bands were transformed to the Hue, Saturation and Value (i.e. HSV) colour model, and the temporal variations of the HSV parameters were compared with some physico-chemical characteristics of the water bodies.
For the study case, it was possible to establish that water bodies with higher electric conductivities reflect more light (higher V values) but their saturation component (S values) are lower. Similarly, S values show a decrease with the occurrence of external factors such rainfall, possible associated with clouds and water vapor. The pH exhibits a relation with the H values, which is related to the chemical composition of water, specifically to changes in Fe(II), Fe(III) and SO4 concentrations. Pit lakes with pH values < 5, display red to orange colors (H values ranging between 0.05 to 0.1), possible associated with the saturation of Fe-hydroxides as calculated by PhreeqC. At pH values > 8.5, tailings ponds display blue to turquoise colors which is associated to both sulfate and dissolved ferrous ions, as well as others divalent metal (such as Cu) that are soluble at these alkaline conditions. At pH values between 5 and 8.5, pit lakes and tailing ponds exhibit orange to green colors (H values ranging from 0.2 to 0.5), which seems to be associated to a decreasing Fe(III)/[Fe(II) + Fe(III)] ratio as pH values increase.
These preliminary results indicate that:
- HSV parameters of different water bodies affected by mine drainage can be obtained using Landsat imagery, particularly on surficial water bodies with sizes greater than the pixel of the satellite images used.
- Temporal variations of HSV parameters can be associated with the physico-chemical characteristics of these water bodies (e.g. pH, chemical composition), and therefore, can be used for monitoring purposes.
