DECLARATION 2- PUBLICATIONS
5.2 Plot Scale Geophysics, Soil Water and Nutrient Dynamics
5.2.1 Geophysics
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ERT measurement is a geophysical method used to characterize the subsurface material (ABEM, 2005) and has gained popularity in hydrological sciences in the past decade due to its ability to profile the properties of the subsurface material and underlying features which influence subsurface hydrology, something which is quite challenging to achieve with classical catchment monitoring networks. Marti and Sabater (1996) identified that parent geology is related to the nutrient uptake within the riverine systems. In particular, Munn and Meyer (1990) found that the ratio of N to P largely determined the uptake of one or other nutrient: streams with a lower N: P ratio (e.g. volcanic parent geology) would have a higher uptake rate of N than P, while those of lower P availability (e.g. granitic parent geology) would show higher uptake of P than N. In other words limitation of a given nutrient would increase its uptake (Valett et al., 1996).
Figure 5.12 displays the subsurface resistivity distribution in a NS direction obtained along the ERT survey W1. Presence of high soil moisture in the top layer that probably connects with a perched aquifer with very conductive material (<100 Ωm) exists near the northern edge of the ERT transect at 6 m below ground level. Compared with hillslope hydropedology transects shown in Section 5.1.2 above, the sandy nature of the soils allowed easy infiltration of rain water, while the soft plinthic horizon acted as the aquitard supporting the perched water table. Drainage was dependent on lateral movement only where an increase in wetness was expected as water moved downslope in a SN direction.
Figure 5.12: Transect W1 located between the Runoff Plots and Flume 1 (Lorentz et al., 2011).
A shallow resistant unit (200-300 Ωm) overlies the perched aquifer and on the downstream end of the field water seepage can be seen on the ground surface. There appears to be a near surface water supply to the waterway as well as a deeper water body, which is likely the same
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as that observed in the borehole (BH), located in the upper headwater catchment of Mkabela.
This creates hydrologic connectivity where surface-subsurface water interaction occurs resulting in nutrients and sediment loads being exchanged. This is corroborated from isotope analyses of runoff water collected from runoff plots (RP1 and RP2) where similar isotope values to those obtained from the borehole (BH) occurred (Appendix K).
Resistivity measurements along transect W2 indicates a ~3m deep sandy and resistive layer (400–900 Ωm) at the middle of the transect overlying a weathered zone comprised of two shallow, perched water bodies (<100 Ωm), one along the northern side that is about 36 m long and 10 m thick and another south of the transect (Figure 5.13).
Figure 5.13: Transect W2 located adjacent to upstream Flume 1 (Lorentz et al., 2011).
These perched water bodies are responsible for holding pre-event water and allow nutrient loads to migrate from the subsurface to the surface where it becomes runoff on the waterway.
This occurs during rainfall events and is demonstrated by a similarity in the analysed isotopes collected at Flume 1, specifically the isotope values in rainfall and borehole water. The hydrological behaviour of this hillslope as explained in Section 5.1.2 is expected to result in the accumulation of water during the rainy season followed by lateral drainage in the saprolite and soft plinthic horizons.
Transect W3 was located along watermarks 1, 2, 3 and 4 in the upper sub-catchment (Figure 5.11). The section traverses the waterway as indicated in Figure 5.14. A sandy layer (200-600 Ωm) covers both sides of the stream. It is underlain toward the west by a perched aquifer (<100 Ωm) at about 4 m depth. Immediately below surface of the waterway there exists unconsolidated and transported sediments (sands) (~290-300 Ωm) that overly deeper leached
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fine clay deposits that have a lower resistivity (~140-200). At the eastern end of the transect, a very dry portion (>500 Ωm) is revealed.
Figure 5.14: Transect W3 adjacent to the nests of soil moisture sensors (Lorentz et al., 2011).
From a hydrologic connectivity point of view, the unconsolidated sand layer on the surface of the waterway has less water retention capacity than the consolidated very fine clay deposit below it (waterway position, Figure 5.14). This means that during a rainfall event, even of low intensity, the thin sand layer on top of the waterway would be easily saturated as it is connected to the clay deposits below. Thus, runoff results much faster on the surface of the waterway than it would upslope of its traverse to the channel (Figure 5.14). At this juncture, much of the water contributed to the stream would be from the sub-surface (pre-event water).
This is probably the reason why the seasonal time series data show that water samples collected from Flume 1 exhibit stable isotopic values similar to those in water from the borehole at the early stages of winter events (July-November). In contrast, isotope values in surface waters differ significantly from those in the borehole later in the year (November- March) as some values began to be similar to those from rainfall (Appendix K).
The resistivity section obtained along transect W4 exhibits three major layers including 3.7 m of sandy soil ( 200-600 Ωm), followed by groundwater bearing saprolite (<100 Ωm) with a water table located at approximately 4 m depth. Sandstone is located at a depth of 20 m depth (Figure 5.15).
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Figure 5.15: Transect W4 located between W5 and the upstream Flume 1 (Lorentz et al., 2011).
The unconfined aquifer in the saprolite layer shown in Figure 5.15 can hold a large amount of water before a water table forms and may occur for one to four months on average during the rainy season (see Section 5.1.2 above). The ramification of this is that leached nutrients from excessive fertilizer applications may find their way to this groundwater source. Once in the groundwater the fertilizer may be hazardous to the environment for a long period because of the vast quantities of water present. It is difficult to clean-up a contaminated groundwater source once pollution occurs.
Transect W5, which is located adjacent to Flume 2, exhibits a conductive thin layer of soil (<70 Ωm) followed by a sandy soil horizon which thickens upslope. A perched aquifer (<100 Ωm) is located in the weathered zone as indicated on Figure 5.16. The bedrock which is fractured sandstone forms a resistive bottom layer (> 200 Ωm). It is located from 2 m to 12 m deep, increasing upslope. This interplay of geologies has important implications for the hydrological processes operating within this catchment, particularly since soil hydraulics will be influenced by the different soil textures and porosities associated with these different geologies (Riddell et al., 2010). Transect W5 reveals the nature of sandy soils which overly deep leached fine clay deposits, which are confined by vertical weathered saprolitic protrusions from the underlying fractured Natal sandstone or bedrock. The significance of this observation is particularly important in the way the aquifer retains water. Further isotope analysis will reveal the nature of the groundwater recharge processes that these weathered zones facilitate within the confines of bedrock controls or fractured Natal sandstone.
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Figure 5.16: Transect W5 located immediately upstream of Flume 2 (Lorentz et al., 2011).