Potential recharge to groundwater estimation, grid dataset, Nulla Basalt Province

Brief description

This grid dataset is an estimation of the relative surface potential for recharge within the Nulla Basalt Province. This process combined numerous factors together as to highlight the areas likely to have higher potential for recharge to occur. Soil permeability and surface geology are the primary inputs. Vegetation and slope were excluded from consideration, as these were considered to add too much complexity. Furthermore, this model does not include rainfall intensity – although this is known to vary spatially through average rainfall grids, this model is a depiction of the ground ability for recharge to occur should a significant rainfall event occur in each location. The relative surface potential recharge presented is estimated through a combination of soil and geological factors, weighting regions that are considered likely to have greater potential for recharge (e.g. younger basalts, vent-proximal facies, and highly permeable soils). Near-surface permeability of soil layers has been considered as a quantified input to the ability for water to infiltrate soil strata. It was hypothesised that locations proximal to volcanic vents would be preferential recharge sites, due to deeply penetrative columnar jointing. This suggestion is based on observations in South Iceland, where fully-penetrating columnar joint sets are more prevalent in proximal facies compared to distal facies in South Iceland (Bergh & Sigvaldson 1991). To incorporate this concept, preferential recharge sites are assumed to be within the polygons of vent-proximal facies as derived from detailed geological mapping datasets. Remaining geology has been categorised to provide higher potential recharge through younger lava flows. As such, a ranking between geological units has been used to provide the variation in potential recharge estimates. Reference Bergh, S. G., & Sigvaldason, G. E. (1991). Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, South Iceland. Bulletin of Volcanology, 53(8), 597-611. doi:10.1007/bf00493688

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Maintenance and Update Frequency: asNeeded

Statement: The input datasets are soil layers and surface geological units. Soil data acquired from ASRIS provided soil permeability information. The near-surface permeability was used as this is the first layer through which water would travel. It was classified into discrete classes, based on the ASRIS handbook (CSIRO 2012, p51), and then scores assigned. Ks1 range Score Description 30-300 8 highly permeable (10 is level above which isn’t here) 3-30 6 moderately permeable 0.3-3 3 low permeable 0.03 -0.3 1 very low permeable <0.03 0 blank (remove?) as impermeable Surface geology, based on the Queensland Detailed surface geology polygons, was considered in several stages. Firstly a mask was used to exclude areas outside the Nulla Basalt Province, namely the areas of outcropping Palaeozoic basement rocks. It was hypothesised that locations proximal to volcanic vents would be preferential recharge sites, due to deeply penetrative columnar jointing. This suggestion is based on observations in South Iceland, where fully-penetrating columnar joint sets are more prevalent in proximal facies compared to distal facies in South Iceland (Bergh & Sigvaldson 1991). To incorporate this concept, it was decided that polygons mapped as having vent-proximal facies (i.e. “/v” in the mapping code) would be provided a weighting score of 7. The remaining geology was incorporated with the thought that recent lava flows retain blocky structures, while weathering has smoothed out the surfaces of the oldest lava flows to begin soil development. As such, older basalts have lower weighting scores compared to the youngest. Within this rating, sedimentary units have been considered: alluvium is weighted higher than most, while lacustrine clays and silts are weighted lower. However most sedimentary units are given a moderate score, with a reliance on the soil Ks values to discriminate units. Standing surface water bodies have been deemed a score of 0, as these are sites of water without geological units mapped; standing water is likely a location of discharge as opposed to recharge. This results in the following geological scoring table: Unit Score Notes Toomba Basalt 10 Most permeable unit as this is the youngest and blockiest lava flow Alluvium (clay, silt, sand/gravel) 7 Assumed to be relatively permeable Yates Creek Basalt 6 Relatively young basalt Colluvium (clay, silt, gravel); Black soil (mix – let Ks sort out) 5 Mixed sediments, using Ks values to discriminate Birdbush Basalt; Sturgeon Basalt; Anabranch Basalt Campaspe Fm 4 Moderate score for other basalts. Campaspe Formation is mixed. Qbn, TQbn, Allensleigh Basalt Lake deposits (clay,silt) 3 Relatively low scores for undifferentiated lavas and old Allensleigh Basalt Lake deposits have low score Tbn 2 Oldest lava flows have diminished score Vents 0 (7) 0 in the general phase 7 in the separate layer Vents are considered in the separate layer Outcropping Palaeozoic units; water bodies 0 Exclusion Apart from the exclusion areas (which were masked out by using a zero multiplier), the various factors were added together for the Nulla. This produced a relative high and low across all the considered factors. There is no unit to this overall score, so there cannot be a direct calculation of the recharge rates. It was considered inappropriate to perform this without sufficient field validation. Reference Bergh, S. G., & Sigvaldason, G. E. (1991). Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, South Iceland. Bulletin of Volcanology, 53(8), 597-611. doi:10.1007/bf00493688

Notes

Purpose
The relative surface potential recharge is depicted as a grid. This model is a depiction of the ground ability for recharge to occur should a significant rainfall event occur in each location, and so does not include the spatial variability of rainfall intensity or amounts. The purpose of this grid is to depict areas within the Nulla Basalt Province that are more or less likely to provide recharge to the groundwater system.