3.1. Daily Groundwater-Level Fluctuations
The daily groundwater-level fluctuations of all four boreholes and daily rainfall data obtained from BOM site location number 066161 [
32], over one year, is shown in
Figure 2. The water table depth (WTD) from ground surface in BH1 could be described as having two major stages—an increasing stage until September and a gradually decreasing stage until the end January. The water table depth was highest in September compared with both the beginning and end of monitoring period. The end of water table depth is higher than the beginning of water table depth by about 219 mm. During dry season (May–September) the water table depth increased rapidly despite having some rainfall events which were of small magnitude. In the case of wet season, the water table depth showed a constant level between September and November and continued with a rapidly decreasing stage during many rainfall events with high magnitudes that occurred in the wet season. The annual variation of groundwater level is 594 mm. As shown in
Figure 3, GWL variations were rapid and erratic. This may be attributed to the lateral groundwater flow conditions. No major recharge structures (such as leaking water and wastewater and large lake) or extraction facilities exist in the region. It can be said that the overall response of groundwater level to rainfall depicts a distinctive pattern for BH1 (
Figure 3a) as the groundwater fluctuation exhibited smooth seasonal change.
BH2 displayed one each of distinct WTD decreasing and increasing. There were few minor fluctuations, especially between August 2018 and January 2019. The main decreasing stage occurred in the period May–July. On the other hand, the main increasing stage occurred in the period July–September (
Figure 3b). During the rainy period, there were large fluctuations in WTD. The gap (
Figure 3b) at the beginning of BH2 groundwater fluctuation was due to the logger failure. The maximum water table depth was achieved in September. The rainfall events with high magnitude in the wet season only increased the water table depth between November to December while it decreased after that. As expected, the water table depth increased in the dry season (July–August). The difference between the beginning and end of monitoring was found to be very small (22 mm), while the difference between the maximum and minimum WTD was 421 mm over the monitoring period. BH2 showed some sharp changes in the WTD during the wet season (September–January). The observed drastic changes in WTD is an indication of rapid responses to individual rainfall events, which is linked to several hydro-geological parameters [
24].
BH3 displayed one each of major increasing and decreasing WTD periods. Additionally, two each of minor increasing and decreasing WTD periods were observed. As expected, increasing WTD occurred during dry periods and decreasing occurred during wet periods. The WTD variation pattern appears to be different to BH2, which did not experience major decreasing or increasing periods. However, the increasing and decreasing periods appears to be similar to that of BH2. The highest water table depth was recorded in September (
Figure 3c), which is similar to BH2. The water table decreased from September till the end but there were occasional increases in the wet season. The difference between the beginning of water table depth and end of water table depth is 61 mm. The annual water table variation is 729 mm.
BH4 exhibited two major increases in WTD, and they were followed by minor decreases. The WTD variations appear to be more erratic in the case of BH4. This can be attributed to the presence of lake adjacent to the monitoring borewell. The sudden increase followed by immediate decrease in WTD in February may be due to the error in the monitoring sensor. The highest water table depth (WTD) was recorded in August, which is the end of dry period. Generally, as expected, the water table depth decreased in the wet seasons. The final water table depth was about 352 mm less than the initial water table depth. Its annual water table variation was 452 mm.
Overall, the WTD fluctuations between all the boreholes varied from 0.4 to 0.8 m. BH3 has the highest annual water table variation followed by BH1 and BH4, while the least was BH2. The groundwater level observed across the four monitoring sites show that different hydrogeological regimes impact the groundwater flow and storage. The results appear to indicate no discernible trend in WTD variations. As seen in
Figure 3, there were drastic fluctuations in the water table depths (WTD) on a daily basis. As shown in the figure, the WTD in BH2, BH3 and BH4 appears to vary quite rapidly, and the variation seems to be similar. Especially, the WTD in BH4 appears to be sensitive to rainfall event. This may be because BH4 is located next to the man-made Wattle Grove Lake (WGL). On the other hand, WTD in BH1 appears to be gradually increasing and then reducing during the high rainfall period.
In terms of rainfall pattern, in the beginning of the monitoring year, there was not much rain. Towards the later stages, there were high rainfall events (between October 2018 and February 2019). Since the daily fluctuations were quite unpredictable, average fluctuations over the month were calculated. These are shown in
Table 2. As shown in the table, the difference in the WTD over each month for each of the boreholes are presented. The table also presents the combined average fluctuations against each of the months.
Figure 4 presents a plot of monthly WTD fluctuations against the monthly rainfall for BH3. The figure indicates that there is a direct correlation between the change in the WTD and the amount of rainfall in the given month. Under low and no rainfall conditions, lateral flow appears to be predominant as a result, the groundwater level goes down. On the other hand, when the rainfall increases, the groundwater recharge becomes predominant and as a result its level increases. This observation applies to all the borewells and justifies the application of Equation (4) for calculating recharges for all the borewells.
3.4. Daily Groundwater Recharge Estimate
Using Equations (1)–(4), a computation table was set up to estimate the daily recharge. As discussed above, for BH1, BH3 and BH4, Δt values for no or less rainfall and high rainfall periods were 61 (April and July 2018) and 62 days (October and December 2018), respectively. For BH2, the corresponding Δt values were 31 and 62 days, respectively. The calculations are summarised in
Table 3. As can be seen from the table, the daily recharge for each monitoring well was estimated as 5.23, 2.70, 1.77 and 1.37 mm/day at BH3, BH4, BH2 and BH1, respectively (
Table 3). Both BH1 and BH2 daily recharge were below 2 mm/day. The low recharge obtained for BH1 and BH2 could be attributed to drier soils with higher moisture holding capacity or high surface runoff. During the dry season (May–September), there were several rainfall events that did not lead to an increase in the water table depth especially having thicker unsaturated zone. The recharge rates estimated for BH3 and BH4 were relatively higher than BH1 and BH2. This may be due to the presence of the Wattle Grove Lake (WGL) in the vicinity of BH3 and BH4. The impact of urban lakes on the groundwater recharge was also reported by other researchers [
47,
48]. Recharge values for BH3 and BH4 indicate that about 2 to 5 mm/day of infiltration may be expected due to the presence of the lake in the area. This can have some significance for managing the stormwater from the urban area.
3.5. Physico-Chemical Analysis of Borewell Water
Results of different physico-chemical analysis is presented in
Table 4. The quality parameters of bore water was compared (
Table 4) with other studies conducted within 25 km radius of the study area and drinking water quality standards [
49,
50,
51]. Average pH values found for the groundwater in BH1, BH2, BH3 and BH4 were within the literature values observed around the study area. On average, the mean dissolved oxygen (DO) values found for the groundwater in all four boreholes exceeded the study areas range. The DO values observed in this study are generally higher and may indicate that there is active inflow of rainwater into the aquifer system. The average total dissolved solids (TDS) value of BH2 was below the catchment range while TDS values of both BH3 and BH4 were within the range. On the other hand, the average TDS value of BH1 far exceeded the range of values obtained for the study area by approximately 36%. The average concentration of Ca, K and Fe of all four boreholes were within the range of values reported for the catchment. In terms of the average concentrations of Mg and Na, only BH2, BH3 and BH4 were within the range reported for the study area while BH1 was outside the range. BH1 average electrical conductivity (EC) values exceeded the values reported for the study area by about 15%. Higher EC values observed for BH1 is an indication that the groundwater may have a saline source. The ionic dominance for freshwater is in the order of Na
+ > Mg
2+ > Ca
+ > K
+.
It should be noted that Denham Court, which is one of the closest sites to the Wattle Grove Lake, appears to show very high concentrations of cations, particularly for Na [
49]. Additionally, this site showed very high TDS. On the other hand, the Glenlee Road and Menangle Park sites, which are bit farther away from Wattle Grove catchment, show significantly lower levels for TDS, Na, Fe and EC. In particular, groundwater in BH1 contained very high levels of Na and these values appeared to be similar to those observed for Denham Court site (
Table 4).
Figure 7 and
Figure 8 show the variation in the groundwater quality in all the four boreholes and the lake using box plots and timeseries graphs, respectively. As shown in the box plots (
Figure 7), the variations in the cation concentrations appear to be significant particularly for BH1 and BH4, this may be attributed to lateral flow of groundwater due to existing hydro-geological conditions and recharge. While variation in the quality of BH4 groundwater may be attributed to the presence of lake, variation in BH1 groundwater quality may be to the hydro-geological conditions, saltwater intrusion or to the presence of some external contaminant sources. Close observation of
Figure 8 indicates that during high rainfall periods, cation concentrations in the lake water appear to drop in comparison to that of groundwater. This reduction appears to be significant in the case of BH2, BH3 and BH4. This may be attributed to the recharge occurring due to the rainfall. Temporal variations as shown in
Figure 8 appear to indicate both groundwater and lake water quality is affected by the rainy periods. This again indicate the recharge of groundwater during rainy periods which, in turn, influencing the groundwater quality.
Another important observation that can be made from
Figure 8 is that during the rainy periods (September–November 2018) sodium and magnesium concentrations in BH1 groundwater increased. This may indicate some hydrogeological processes that are taking place near BH1 during rainy periods.
3.6. Variation of Groundwater Quality and Its Suitability as Drinking Water Supply
The pH value of samples in BH1, BH2, BH3 and BH4 were in the ranges 5.8–7.3, 5.1–7.1, 5.3–7.2 and 4.3–6.6, respectively (
Figure 7). The average pH values of analysed groundwater samples of BH1, BH2, BH3 and BH4 were below the recommended minimum acidic value of 6.5 by WHO [
50] and ADWG [
51]. Both BH2 and BH4 had about 92% of the samples below the minimum safe prescribed limit for drinking water, followed by BH3 with about 85%, while BH1 was 62%. In general, the groundwater of the study area could be described as slightly acidic. The acidic nature of water could be as a result of the high mineral rich rocks making up the aquifers. The minimum and maximum values of turbidity obtained in all the four monitoring boreholes exceeded the recommended limit of 5 NTU by World Health Organisation (WHO) and Australian Drinking Water Guidelines (ADWG). BH1 groundwater is the most turbid, followed by BH3 and BH4, respectively. High levels of turbidity are an indication of possible presence of contaminants. If the groundwater is to be used directly for drinking, it shows all four are unsuitable as they exceeded the turbidity limit of 5 NTU limit [
50,
51].
The minimum value of EC in BH1 groundwater, as given in
Table 4, exceeded both the minimum and maximum values of EC in BH2, BH3 and BH4, as shown in
Figure 7 and
Figure 8. The average EC of BH1 groundwater was 15,530 µS/cm. BH2 with average EC of 550 µS/cm was the least. Using the average EC values to classify the boreholes, BH1 is highly saline, BH3 and BH4 are moderately saline, whereas BH2 is slightly saline [
52]. Higher EC in the groundwater of BH1 indicates that there could be some localised sources of contamination that has resulted in higher EC levels such as fluid migration into the aquifer from nearby formations. Additionally, it can be due to saltwater intrusion as it is close to an estuary (about 10 km). The high salinity in the groundwater in the Wianamatta Group of formation is due to the high proportion of soluble salts of marine origin available for dissolution, leaching and mobilisation [
39]. Moreover, significant increase in concentration of dissolved solids and the presence of metallic ions may be responsible for high EC [
53]. Enrichment of salt because of evaporation effect and leaching also cause high level of EC in groundwater [
54]. When a source of drinking water becomes more saline, it is expensive to provide potable water both in terms of capital and operating costs.
The TDS concentrations of BH1, BH2, BH3 and BH4 were in the ranges 12,082–15,974 mg/L, 277–613 mg/L, 419–1550 mg/L and 2042–2590 mg/L, respectively. The WHO recommends TDS concentration of 600–1000 mg/L for drinking water purposes; the minimum TDS value of BH2 and BH3 were below WHO guidelines, while only the maximum TDS value of BH2 was meeting the guideline value. In addition, 82% of BH2 groundwater samples analysed were below the minimum 600 mg/L guideline value, while the remaining 18% were within the 600–1000 mg/L WHO threshold limit. A total of 50% of BH3 groundwater samples analysed were the minimum 600 mg/L, 30% was above the maximum 1000 mg/L prescribed limit, while 20% was within the recommended WHO range. Both BH1 and BH4 recorded 100% of samples that were above the maximum 1000 mg/L limit. Therefore, the groundwater collected from BH1 and BH4 are unfit for drinking without adequate treatment. BH1 samples exhibited a higher standard deviation, which suggests local variation in point sources, soil type, and multiple aquifer system [
54].
The abundance in cation ranges from Na followed by Mg, Ca and K, respectively. The concentrations of these cations in groundwater are usually greater than 1 mg/L [
55,
56]. The Na concentration of BH1 groundwater ranged from 3034 to 4047 mg/L, with an average value of 3654 mg/L, BH2 varied between 66 to 202 mg/L, with an average of 138 mg/L, BH3 was within 15 to 304 mg/L, with an average of 214 mg/L, while BH4 ranged from 15 to 107 mg/L, with an average value of 85 mg/L, respectively. Some of the possible reasons for the high Na observed in BH1 compared to others could be due to the saltwater intrusion or some local source. Saltwater intrusion could be a likely cause as the nearest estuary is about 10 km from the catchment area. However, this needs to be further investigated. According to WHO standard, the maximum permissible limit (MPL) for Na is 200 mg/L [
50] and BH1 minimum and maximum values exceeded this value. Groundwater samples collected from BH2 and BH4 met WHO Na concentration requirement for drinking purposes whereas 60% of analysed groundwater samples from BH3 exceeded it.
The second dominant cation is Mg and WHO prescribed limit for Mg in drinking water is 100–300 mg/L. The Mg concentration of the samples analysed from BH1 ranged from 562 to 840 mg/L, BH2 ranged from 11 to 40, BH3 ranged from 13.18 to 20.45 and BH4 was 95.3 to 106.7 mg/L. As per WHO minimum prescribed limit of 100 mg/L for Mg concentration in drinking water, both BH2 and BH3 were below the limit. On the contrary, BH1 exceeded both the minimum and maximum permissible limit (100–300 mg/L). About 50% of the samples collected from BH4 were below the minimum permissible limit while the remaining 50% were within the WHO range of 100–300 mg/L [
50]. The WHO guideline value for Ca concentration in drinking water is 100–300 mg/L. The groundwater samples collected from BH1 did not exceed the maximum limit. For the rest of the boreholes, Ca concentrations were below the minimum permissible limit, which is 100 mg/l. Both Mg and Ca ions contribute to water hardness but do not pose any health threat [
50].
Season has an influence on groundwater quality variability with respect to iron. The concentration of iron in groundwater during rainy season is higher than during dry season. This could be attributed to influence of rainfall infiltrating and dissolving mineral in rocks and soil which are leached into groundwater sources [
56]. The average values and standard deviation values of iron for each borehole are presented in
Table 4. The ADWG and WHO recommended guideline value of iron for drinking water purposes is 0.3 mg/L. However, all four boreholes exceeded iron guideline value. The metabolic activity of bacteria impacts on the concentration of iron found in groundwater. Iron concentrations above the 0.3 mg/L value may produce bad odour, colour, scaling and corrosion. High iron concentrations are commonly found in shallow wells of less than 30 m deep than in deeper wells [
51,
57]. Water that contains iron does not have any harmful effect when consumed by human beings. Long term consumption of drinking water with high iron concentration could cause liver disease. Communities can reject groundwater as a source of water supply when the water is coloured due to high iron concentration [
56].
3.7. Groundwater Quality for Use as Irrigation Water
Irrespective of the sources of irrigation waters, they still carry certain chemical substances in solution, dissolved from the rocks or soils over which the waters have passed. The quality of irrigation water for use is determined by the concentration and nature of these dissolved constituents [
57]. The suitability of groundwater for irrigation purposes is directly linked to the effect of mineral constituents of water on plants and soil. The irrigation quality of groundwater is assessed by the total salt concentration, which is measured by electrical conductivity (EC), sodium percentage (% Na), residual sodium carbonate (RSC), permeability index (PI), sodium adsorption ration (SAR) [
58,
59] and Kelly’s ratio (KR) [
60,
61]. Irrigation waters are classified based on the concentration of substances in it. EC is a good indicator of the salinity hazard used in classifying water for irrigation use [
58]. However, in this study, SAR and % Na which are widely used were adopted to assess the suitability of Wattle Grove groundwater for irrigation purposes.
The Wilcox diagram [
58] was used to evaluate the boreholes water suitability for irrigation purposes (
Figure 9a). The SAR values obtained from the boreholes were analysed in accordance with US Salinity Laboratory diagram (USSL) [
45] (
Figure 9b). As per USSL classification, BH1 groundwater is not suitable for irrigation (
Figure 9a). This classification is also reflected in Wilcox diagram (
Figure 9b). The groundwater collected from BH1 is classified as C4–S3, indicating extremely high conductivity and salinity (
Figure 9b). C4 is not recommended for irrigation under ordinary conditions but can be used sparingly under very special circumstances [
58]. Prolonged use of saline water for irrigation can lead to saline soils [
62].
In the case of BH2, about 93% (
Table 5) of the samples fell under C2–S1, and the rest as C1–S1 (
Figure 9b). C2 can be used when a moderate amount of leaching occurs. However, since the groundwater is of low salinity (S1), it can be used to irrigate many soils without causing developmental issues associated with high level sodium concentration. This is also reflected in the Wilcox diagram (
Figure 9a), whereby approximately 80% of the samples collected from BH2 were found in the class of permissible to doubtful, whereas 20% of the samples were in the class of excellent to good for irrigation.
SAR of BH3 groundwater revealed medium, high and very high values, whereas the EC appears to be relatively stable under C3 (
Figure 9b). Generally, BH3 groundwater can be used to irrigate plants with moderate salt tolerance under very special conditions. As per USSL classification, about 73% (
Table 5) of the groundwater samples of BH3 were in the class of doubtful to unsuitable, while the rest were permissible to doubtful for irrigation purposes. These results indicate that the groundwater may not be suitable for irrigation. If used, it should accompany salinisation mitigation plan.
Finally, suitability of BH4 groundwater is classified as C2/C3–S1/S2. The value of S2 can present sodium hazard in fine-textured soils of high cation-exchange capacity under low-leaching conditions but may be used on coarse-textured soils that have good permeability. The implication of its very high conductivity and medium salinity is that it can be used to irrigate plants on coarse-textured soils but under very special circumstances [
61]. In the case of BH4 groundwater samples about 80% fell in doubtful to unsuitable category for irrigation purposes and the remainder were in the class of permissible to doubtful (
Figure 9a,
Table 5).
The above results indicate that the groundwater is generally unsuitable for irrigation without appropriate treatment. However, the lake water appears to be excellent for irrigation as per both USSL and Wilcox classifications (
Figure 9).
3.8. Hydrogeochemical Analysis
Gibbs plots indicate the evaporation dominance in BH1 and few BH4 groundwater samples, whereas BH2, BH3 and some BH4 groundwater samples shows dominance of rock-water interaction (
Figure 10a). The average (Ca
2+ + Mg
2+)/TZ
+ ratio (where TZ
+ is total cations) at BH2, BH3 and BH4 was 0.2, indicating high abundance of sodium and potassium ions and it justifies the silicate weathering in the study area, whereas average (Ca
2+ + Mg
2+)/TZ
+ ratio for BH1 was 0.5 indicating a stable hydrogeochemical weathering (
Figure 10b) [
8]. It may be noticed from the plot of calcium and magnesium versus total cations (
Figure 10b) that the data points lie below the 1:1 line and it is more prominent at higher TZ
+ concentrations. Similarly, at higher cation concentrations, (Na
+ + K
+) concentration appears to dominate cations (
Figure 10c). This is characteristic of the Wianamatta group [
39]. Moreover, Mg
2+/Na
+ vs. Ca
2+/Na
+ bivariate plots show that groundwater clusters are within the range of global average silicate and evaporites zone (
Figure 10d). Therefore, from the above geochemical data, it can be concluded that the dissolution/weathering of silicate rock acts as a major contributor for Na and K ions in BH2, BH3 and BH4, whereas evaporites are responsible for the enriched dissolved solids in the groundwater of BH1.
3.9. Correlation Analysis
The physico-chemical parameters and water table elevations were analysed using the SPSS Statistical software to calculate Pearson’s correlation coefficient (r). This analysis was carried out for each of the boreholes as well as for the combined data. The study indicated that the correlation analysis for individual boreholes did not show any specific discernible interactions. However, the combined correlation analysis appears to indicate significant correlation between EC, TDS, Turbidity and pH (
Table 6). There is a significant positive (green box in
Table 6) correlation between turbidity and TDS. The turbidity of the groundwater was observed to be significantly high, which is in the range of 33 to 530 NTU. This means that the turbidity may be caused by the colloidal particles generated by the salt precipitate.
Sodium is found to be having positive correlation with dissolved oxygen (DO). This means that when the DO is higher, there is increased sodium concentration. Higher DO occurs due to increased recharge during the rainfall events. This can be related to the possibility of sodium leaching into the groundwater when recharge occurs. This may be due to the rise in the groundwater table, which may be helping the dissolution of the sodium-based salts present in the soil. Additionally, it can be noted that there is a significant positive correlation between pH, Mg, K and Ca.