Fertilizer application experiments

Two fertiliser application experiments were performed; for both experiments the synthetic fertilizer KAN (calcium ammonium nitrate) was used in accordance with the appropriate agricultural practice.

For the meadow with two mowings an application rate of 40 kg N/ha was used, therefore for 150 m² of test area (Fig. 3) 0.6 kg N was required. Regarding 28% N portion in KAN, 2.25 kg of that fertilizer was used. Electrical conductivity, temperature and pH were measured in situ in water samples during the experiment (Fig. 4). The water samples were also examined in the laboratory for nitrate ion and nitrite ion concentrations and nitrogen isotope ¹⁵N.

0

FIG. 14. Deuterium isotope composition of water in sampling points MP1–MP10.

0

precipit. MP1 MP2 MP3 MP4 MP5

FIG. 15. Deuterium excess in sampling points MP1–MP5 (vertical dashed line: injection, horizontal dashed line: d-excess for Sinji Vrh unsaturated zone).

0

precipit. MP6 MP7 MP8 MP9 MP10

FIG. 16. Deuterium excess in sampling points MP1–MP10 (vertical dashed line: injection, horizontal dashed line: d-excess for Sinji Vrh unsaturated zone).

TABLE 4. THE FIRST APPEARANCE OF DEUTERIUM IN DIFFERENT SAMPLING POINTS AND THE APPEARANCE OF MAXIMUM ²H COMPOSITION

Appearance of tracer

(days) Max. ²H composition (‰)

Max. value of

d-excess (UNZ SV) Appearance of Max. value (days)

In the first fertiliser application experiment (experiment G1) nitrate appeared in water samples of almost all sampling points after approximately 30–35 days (Fig. 17), which was five days after the heavy rain (4 July; 51.7 mm), followed by more or less constant rain [23]. The first precipitation event (Fig. 17) caused dissolution of the fertiliser and transported it into the soil zone. The highest nitrate concentration (15.1 mg/L) appeared at the measuring point MP5 (Fig. 17). Previous tracer experiments and structural mapping identified this sampling point as a fast channel with the potential to permit large contaminant fluxes (see Fig. 2: tracer test sampling area). Natural background unsaturated zone concentrations measured in springtime, which are dominated by mineralization processes in the soil, were higher than the observed unsaturated zone nitrate concentrations following fertiliser application.

The second fertiliser application experiment (G2) was performed in order to obtain further information on nitrate percolation through the soil cover. For this reason, suction cups were installed in the soil and isotope analysis was carried out on water obtained from multiple sampling points (MP2, MP5, MP10 and MP21 in Fig. 18). The nitrate appeared one week after one very large precipitation event (68.8 mm; 21 Sept.), about 22 days after fertiliser was applied. Since the isotope composition of the synthetic fertilizer KAN is about 0‰, the isotope data ¹⁵N (Fig. 18) confirmed that the nitrate source was the fertiliser applied on the meadow in experiment G2 [23]. The nitrate transport along the fast channel at measuring point MP5 was showed the first breakthrough of nitrate after 8 days. In some measuring points another flush of the nitrate was detected (MP21 and MP10). After another very large precipitation event in the following spring (1 March; 85.1 mm) the nitrate was flushed again, which released nitrate previously retained in microfractures of the unsaturated zone (MP2 in Fig. 18).

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FIG. 17. Precipitation events and nitrate concentrations (detection limit: 4.4 mg/L) in sampling points MP5 for experiment G1 with δ¹⁵N data for MP5.

FIG. 18. Precipitation events and nitrate concentrations (dashed horizontal line: detection limit at 4.4 mg/L) in sampling points MP2, MP5, MP10, MP15 and MP21 for experiment G2 with ¹⁵N data.

5. CONCLUSIONS

Fractured and karstified rocks are highly heterogeneous and complex, therefore knowledge of the rock structure is of paramount significance for predicting flow and transport. The transport of pollutants depends on i) saturation rate of the soil and unsaturated zone, ii) on precipitation events and iii) on the presence of channels and interconnected fracture networks. Pollution remains in the fractures of the upper part of the unsaturated zone and is flushed by subsequent large precipitation events which can occur several months or years afterwards. The degree of flushing is highly dependent on the antecedent moisture conditions within the unsaturated zone.

If there has been no precipitation prior to the application of a tracer then the first observed point of breakthrough will be the maximum concentration measured. Modeling such breakthrough curves can be problematic.

The results of isotopic investigations gave additional information about the transport processes in the unsaturated zone. Results of ¹⁸O and ²H isotope composition provided a better understanding of the research area and provides guidelines for further long-term and short-term monitoring strategies.

The tracer experiment has shown different flow velocities. Karst conduits or large fractures/faults show no storage capacity, the most rapid flow, and very limited dispersion. Broken and fractured zones have some storage capacity, which leads to retardation and the slower flow. The fractured and karstified rock could be seen as discrete conduits within highly fractured rock. The latter could be considered as a permeable matrix, since the rock (limestone) matrix porosity (intracrystaline) is negligible and the system could be best modelled with a hybrid model (comprising discrete conduits and a continuum of dense fractured system) rather than a dual porosity model (where there would be a continuum of conduits and a continuum of dense fractured rock). This is a subject for future research.

Results of the fertiliser application experiments have shown that a thin autochthonous soil cover on karstic rock is insufficient to retain nitrate and prevent pollution of groundwater. However, the maximum nitrate concentrations (up to 16 mg/L in G1 and up to 36 mg/L in G2) were lower than natural background values and the drinking water limit. Therefore the usage of fertilisers should be in accordance with strictly defined standards. Further detailed studies on this aspect are needed within the frame of the research into karstic aquifer vulnerability. Additional experiments with an emphasis on biogeochemical transformations (e.g. measurement of nitrogen gas, N₂O, soil nitrogen compounds, dissolved nitrogen compounds including isotopes, and microbiological processes) is planned in order to better understand the behaviour of nitrate in soil and the unsaturated zone.

ACKNOWLEDGEMENTS

We acknowledge the financial support from the Ministry of Science and Technology of the Republic of Slovenia for research projects under the research grants No. J1-7172-94 and J1-0462-98, for doctoral scholarship for Barbara Čenčur Curk (S21-210-005/15249/97) and Branka Trček (S21-215-007/13034/97) and for joint projects with Ministry for Physical Planning of the Republic of Slovenia, under the research grant No. V2-8575-96. This work was supported also by the International Atomic Energy Agency under the research grant No11519. We express our sincere thanks to Dr W. Stichler (GSF, Germany) for analyses of ¹⁸O and ²H and his valuable advice.

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BEHAVIOUR OF NITROGEN IN THE UNSATURATED ZONE IN Groundwater Science Research Group, NRE/CSIR, Stellenbosch, South Africa

ABSTRACT

Large areas in southern Africa have high levels of nitrate (>50 mg N/L) in groundwater caused either by natural nitrogen fixation or by pollution. Certain environmental conditions such as episodic groundwater recharge enhance such high levels and increase its variability. Soil samples were taken from two entirely different sites to identify sources and transport of nitrate through the soil. In arid Botswana, occasional high nitrate pulses with distinct pollution ¹⁵N signatures are observed in groundwater following extreme rainfall events. These are thought to result from a combination of regular recharge along preferential flow paths and rapid recharge through unleached soils during episodic recharge in this area. Soil samples down to 3 metres in different environments showed the usual vertical decrease of nitrate in both polluted and natural environments.

One profile collecting outwash from a large polluted area showed an increase of nitrate and chloride with depth and represents a situation where episodes of rapid flushing of pollutant nitrate towards the water table may be expected. In humid sandy soils near Cape Town, South Africa, a comparison was made between soils from a reference area covered by leguminous vegetation and from an adjacent agricultural land previously fertilised with sewage sludge. The soil in the reference area was drier due to deeper moisture withdrawal by trees. In the agricultural land constant ammonia levels occur throughout the depth profile while nitrate levels decrease with depth. ¹⁵N levels in ammonia increase towards the water table, while ¹⁵N levels in nitrate remains practically constant with depth. These profiles show the recovery effect of the vadose zone after sludge deposition on the surface was discontinued. Considering both profiles, the results show the different responses that are possible under widely varying conditions of recharge and pollution load and link up with the chemical and isotope content of groundwater.

1. INTRODUCTION

Nitrogen in the form of nitrate is becoming a significant pollutant in groundwater in many parts of