Mining uses large quantities of water in the processing of raw material. Management of effluent generated from this process is important. Legislation prevents companies from altering levels and quality of ground water through the disposal of effluent. An efficient method of management is to utilise vegetation as hydraulic pumps to ameliorate effluent used as irrigation for plantations.
To ensure maximum pumping capacity the irrigation rate must be matched (with a safety margin) to the rate of extraction of the plantation. This ensures maximum effluent disposal with minimum environmental exposure caused by irrigation runoff into rivers & streams and through drainage into aquifers that are often used by urban centres for drinking water supplies.
A mixed species trial plantation of 170 ha was established with three species, Acacia saligna (Acacia), Tamarix gallica (Tamarix) & Prosopis pallida (Algarrobo). Species were known to perform well under the local environmental conditions and considered to be high water users.
Species were installed with Heat Ratio Method (HRM) Sap Flow sensors to monitor water use. Measurements were conducted over a full 12 month period to determine water use rates and evaluate seasonal effects on total water use for each species.
Data were automatically corrected and converted from Raw Heat Pulse Velocity to Sap Velocity and volumetric Sap Flows then, analysed graphically in both 2D and 3D using Sap Flow Tool Software to interpret seasonal variations.
Mean Acacia Water use = 23.6 L/day Mean Tamarix Water use = 9.9 L/day Mean Algorrobo Water use = 4.2 L/day
Stems per hectare = 1000 stems
Acacia Water use = 1,000 x 23.61 = 0.0236 ML/ha/day Tamarix Water use = 1,000 x 9.9 = 0.0099 ML/ha/day Algorrobo Water use = 1,000 x 4.2 = 0.0042 ML/ha/day
Sap Flow measurements determined the water use rates of the three species trialled. Acacia saligna used almost 2.5 times more water than Tamarix gallica and more than 5.6 times that of Algorrobo Prosopis pallida. Based on these measurements an Acacia saligna plantation size of 192 ha is required to pump the current mine effluent of 4.5 ML/day. Based on current rates a minimum 13% increase of the plantation is required. This Sap Flow Data can now also be used to accurately design a new plantation to meet the increased effluent generated by the proposed expansion of the mine.