The occurrence of ponds in the Rio Claro Formation is a unique geomorphologic feature of the region, where large-scale agriculture dominates the landscape. To understand the hydrogeology in rural areas with such conditions, an area containing a pond in a low topographic position surrounded by sugarcane and eucalyptus plantations at a higher topographic elevation was investigated. An initial potentiometric surface map was generated in March 2013 (Fig. 3a) indicating groundwater flow directions to the northwest, coming from the pond towards the high topographic area, preliminarily showing a pond recharging behavior. This hydraulic behaviour generated a series of research questions since a pond at lower topography, which is hydraulically connected to the local shallow aquifer, would typically be considered a potential discharge area.
The first hypothesis describes that the behavior is related to the eucalyptus' high capacity to absorb water. Mattos et al. (2019) assessed recharge before and after converting a pasture cover to a eucalyptus plantation, from 2004 through 2016, over an outcrop area of the Guarani Aquifer System in southeastern Brazil in the same climatic conditions. They concluded that the change in crop type leads to an increase in evapotranspiration and a decrease in recharge. Several species of eucalyptus are able to extract large amounts of water, known for drying out flooded areas, especially when precipitation reaches levels below 400 mm/yr (de Paula, 1999). It is also known that the faster the tree growth and the larger its leaf area, the higher the groundwater consumption (Poore and Fries 1985). In eucalyptus, the growth rate is considerably accelerated up to 7 years, where it approaches its optimal cutting age. It is estimated that a plantation at this age has an evapotranspiration rate of around 800 to 1,200 mm/yr (Foelkel 2005). With recharge being the process in which water infiltrates and reaches a saturated zone, the eucalyptus roots act as a barrier preventing this infiltration, absorbing part of the soil moisture. When soil moisture is no longer enough for the crop, the roots grow down into the soil to find water and nutrients, sometimes reaching the saturated zone. Bernardino et al. (2016) found fine roots inside the monitoring wells with a screen at a 5 m depth in the saturated zone. Previously, Robinson et al. (2006) also observed such an event, surveying spatial patterns of soil water depletion by eucalyptus sp. in Australia. Their results showed that a 7 year old tree could exploit water from soil up to 8–10 m of depth, with a lateral influence of 15–42 m. Christina et al. (2011) reported a depth of 9.5 m at 1.5 year and 15.8 m at 3.5 years in Brazil.
Sugarcane plants have different root dynamics, even though it presents high evapotranspiration rate (~ 830 mm – Cabral et al. 2012) as eucalyptus. Its roots have a much more surface range area than those of eucalyptus. Evans (1936) observed that sugarcane roots could penetrate to depths exceeding 6 m; after, Blackburn (1984) showed that modern agriculture of sugarcane might not follow the same root patterns observed decades ago, showing that most of the roots remain at 1 m depth and only thin roots are found up to 4 m depth. This information associated with stable isotopes of groundwater with depleted δ2H and δ18O values may indicate that sugarcane could reduce infiltration during dry periods, and only humid months result in recharge, reflecting more depleted values under the sugarcane area.
So, analyzing the groundwater stable isotopes samples from the area, firstly, there was a clear linear regression trend indicating an evaporation signature or a mixture between groundwater and pond waters, especially in wells at the eucalyptus area (yellow and red dots plotted right below meteoric water lines, and proportion mixing line, Fig. 5). Secondly, samples of sugarcane (blue dots – more negative values) aligned with the meteoric water line, but not with the mixing line, due to its more negative values, explained by the groundwater flow direction, since the water from the pond does not pass through the sugarcane area (Fig. 3). Lastly, the distribution of groundwater stable isotope signatures indicates that the greater the distance from the pond, the more isotopically depleted is the groundwater, suggesting that infiltrated water receives depleted rainwater isotopes while it moves away from the pond. In such transient conditions, it should be considered that surface water mixes with groundwater, resulting in distinct signatures along the route to the discharge area (the spring) (Fig. 5 and Table 2). Therefore, groundwater stable isotope samples are aligned under global and local meteoric water lines (Fig. 5), suggesting that water from different monitoring wells contain a mixture of groundwater and evaporated pond water. These findings corroborate the idea that the pond acts as a recharge area, and the discharge zone is located near the spring, in the northwest direction (Fig. 2). The aquifer is also recharged via excess water from the local rainfall and varies according to the land uses, occupied by the cultivation of sugarcane or eucalyptus, in this case.
Regarding the relative soil moisture level, it was determined by geophysical resistivity surveys at the eucalyptus and sugarcane plantations; and at the road to the pond (Fig. 4). Since the eucalyptus trees were removed in August of 2013, it was possible to compare soil moisture before (June) and after (September) this event. The fact that the rain has decreased since May 2013, situations were observed: (1) an overall increase in soil resistivity, more likely in humid areas, near the pond (50%), sugarcane plantation (25%), and eucalyptus (76%), suggesting that the lack of rain until September yield at least 50% of the resistivity increase in situations of no soil use alteration. (2) in the eucalyptus area, after the cutting, a disproportionate increase both in values of soil resistivity and impacted area was noted, more than those observed in sugarcane, uphill and downhill areas. On the other hand, (3) the uphill area showed a high resistivity range in June 2013, and changes caused by the dry period were not detected (16%), despite the increase of the affected area.
Regarding the geochemical data, eucalyptus groundwater samples showed overall lower ion concentrations than sugarcane, indicated by low values of EC (Table 2). Sugarcane samples presented higher ion concentrations in most groundwater samples, with calcium generally above 10 mg/L. The average sodium and potassium concentrations were relatively low compared to other ions, except in well S4, which had high potassium (up to 12.4 mg/L) and nitrate (up to 272.6 mg/L), with a low concentration of phosphate (< 0.04 mg/L), which could be related to the use of fertilizers NPK (ratio 20:05:20). Additionally, the shallow reach of sugarcane roots makes it difficult to take nutrients at depth. In other words, mobile ions, like nitrate, can escape from the root influence and reach the aquifer. The pond presented bicarbonate-calcium-type waters with low nitrate and other ion content and intermediary values of EC. Interestingly, the pond water presented the lowest average EC comparing to eucalyptus and sugarcane groundwater. If the pond were in a discharge area context, EC's values would be higher, indicating the contribution of more mineralized water from the aquifer, as pointed by Oliva (2006) in the geochemical model of the Rio Claro Aquifer.
According to Tóth (1963), there is a hierarchical pattern of groundwater flow systems, classified as local, intermediate, and regional. A local flow system develops between high topographies (recharge areas) and low topographies (discharge areas), while an intermediate system consists of several low topographies intervening between recharge and discharge areas. A regional flow system has its recharge area at the highest part of the watershed and its discharge area at the lowest part of the basin. The pond is inserted within this local flow system as a recharge area, not a discharge area.
Based on these findings, in corroboration to the potentiometric surface maps and the isotopic, geochemical and geophysical data, it is possible to affirm that: 1) the eucalyptus area does not have enough influence to change the dynamic of the aquifer hydraulically; 2) the study area is inserted in an intermediate flow system, influenced by the main drainage (spring located at NW of the map – Figs. 2 and 3) that commands the general hydraulic flow and with lower hydraulic heads, indicating that; 3) the pond is inserted within this system as a recharge area, not discharge (see the schematic hydrogeological conceptual model on Fig. 8).