Recharge systems for protecting and enhancing groundwate

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TOPIC 2
Geochemistry during infiltration and flow
These processes are as follow:
A: discharge from sandstone bedrock as (a) seepage into permeable sands and gravels at base of the alluvium, (b)
surface discharge from springs in outcropping bedrock, and (c) seepage along upper sections of stream courses;
B: recharge of low salinity water from (a) streams during periods of flow, and (b) basalts on surrounding ranges as
throughflow to stream headwaters;
C: discharge of deep basin waters (Great Artesian Basin) from faults at base of alluvium, or from exposed bedrock.
These waters are cool, but contain abundant CO 2 ;
D: alluvial groundwaters that have experienced evaporative concentration, usually towards the edges of alluvial
plains or at shallow depth.
Of note is the overall low permeability of the shalower profile of the alluvium, due to high silt content. Low infiltration
rates were determined for surface alluvial material in the central parts of the valley (e.g. Ellis and
Dharmasiri, 1998). This finding supports the conclusion that recharge to the alluvium is dominantly along the
drainage system. Flow of groundwater through the sandstones is relatively slow, and recharge to these foramtions is
distant to the immediate valley (e.g. Dharmasiri et al., 1997).
Definition of proportion of recharge
To assist in the future management of the groundwater resources better definition of recharge processes will be
of value. Although the forms of recharge are relatively well understood, no quantification of amount has been
reported. As shown above water chemistry and isotope character can reflect many processes. Further consideration
of over 100 samples from throughout the valley indicated some merit in a plot of log TDS (total dissolved solids,
mg/L) and δ 2 H (‰).
Figure 7 (upper) displays a diamond-shape field consisting of four end points and composed of two ternary plots.
The end points are the representative values for different water types and are based on 2–4 samples. The left triangle
ABC is the field for sandstone-stream-deep basin sources; the right triangle ADC is the field for sandstone-streamevaporation.
The two triangles are divided by lines of 10%; the position within each triangle enables the relative
proportion of each source to be calculated. For the ADC triangle the endpoints are stream waters, sandstone waters
and groundwaters influenced by evaporation; as the latter parameter is relative fields of low, medium and high are
identified.
In Figure 7 (lower) eight examples of groundwaters from a variety of bores throughout the valley have been
assessed to test the method.
Results are summarised in Table 1, showing the indicated percent recharge from each primary source, as well as
TDS (mg/L). It is clear that bores with a larger component of sandstone groundwater input are more saline. These
results are compared to Cl/HCO 3 : bores with a deep basin component typically have very low values, however, low
Cl/HCO 3 values can also indicate samples more proximal to rainfall recharge areas near to stream headwaters (e.g.
sites 7 and 8).
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin