Abstract

Key Words

Geochemical conditions, Eh, DO, DNRA, DOC

Introduction

Contamination of groundwater by nitrate is a widespread problem in agricultural areas such as the lower Burdekin in the north Queensland where 80, 000 ha is under irrigated surgarcane and other crops. Nitrate concentrations above the Australian New Zealand Environment and Conservation Council (ANZECC) guideline for long-term environmental sustainability (5 mg NO₃-N/L) have been reported in 49 out of the 397 sample bores in the Burdekin region (Weir 1999). Also, during 2000-2002, sampling of ten farmer production bores in the Delta found nitrate levels from 1 to 12 mg/L NO₃^--N (Charlesworth and Bristow 2004). As this area overlies major groundwater supplies and is close to environmentally sensitive wetlands, waterways and the Great Barrier Reef Lagoon (GBR), its impacts on the water quality are under increasing scrutiny. Consequently, there has been increasing interest in understanding any natural processes that utilise the groundwater nitrate and conditions that enhance the processes. While denitrification has been intensively studied with respect to surface water/sediment interfaces, soil environments and waste water treatment processes (Barton et al. 1999) it has in recent years been recognised for its ability to eliminate or reduce nitrate concentrations in groundwater (Postma et al. 1991; Korom 1992).

Denitrification is a microbially mediated process where nitrate is converted to N₂ gas when O₂ becomes limited. While organic carbon is the most preferred electron donor as it yields the most energy to bacteria (Starr and Gillham 1993) other reduced inorganic substrates such as ferrous or pyrite (Postma et al. 1991) have been reported as electron donors in nitrate reduction process. Nitrate is used as an electron acceptor by the bacteria only in the saturated zone. Following equations describe the denitrification process using different electron donors,

	(1)
	(2)

The N₂ gas produced from denitrification processes remains in solution until the groundwater discharges to surface-water and equilibrates with the atmosphere. Measurement of excess N₂ in groundwater, electron donors and other geochemical conditions can give an indication of whether an aquifer has potential for natural denitrification, and to some extent, the fate of nitrate in groundwater can be predicted. While there has been ample observational evidence of iron in groundwater of the lower Burdekin in certain areas including yellowish-red deposits around pump outlets, and some measurements on the distribution of iron (Kelly 1974) in groundwater, no studies have linked iron and/or dissolved organic carbon (DOC) and hydrogeochemical conditions with the nitrate in the groundwater. The objective of this study was to determine the potential for denitrification in the Burdekin aquifer and the effect this might have on the amount of nitrate likely to reach the GBR lagoon.

Methods

The lower Burdekin is located approximately 90 km southeast of Townsville in north Queensland. The Burdekin River Delta aquifer comprises sedimentary deposits extending upto 100 m deep and overlies a predominantly granitic basement. The nature of sedimentation is very complex comprising a mixture of interbedded gravel, sand, silt, mud and clay. The groundwater system in the lower Burdekin is generally considered to be unconfined, due to the presence of sandy to loamy soils and the discontinuous nature of underlying clay layers (Brodie et al. 1984). Groundwater flows generally from the south to the north based on potentiometric surface maps. Water loss from the aquifer is through discharge to the sea and river, pumping for irrigation and evaporation. Fertiliser nitrogen recommendation for sugarcane in the Burdekin is 160-220 kg of N/ha/yr and is usually applied as urea in a single operation at the start of the season (April-October).

Figure 1. Sampling locations during September/ October 2003 and in January 2004

Groundwater sampling of monitoring bores installed by the Queensland Department of Natural Resources Mines and Energy was performed in two different periods, one during September/October 2003 and the other during January 2004. In the first period, 28 single and 2 nested bores were sampled and during January 2004, 22 single and 5 nested bores were sampled (Figure 1). Most of the chosen bores were confined to areas planted sugarcane. The depth of the bores varied from 5.4 m to 88.5 m. Water samples were withdrawn from bores using a stainless steel submersible pump with a Teflon discharge line. Purging of a bore continued until either three bore volumes had been pumped out or until the pH had stabilised. Redox potential (Eh), pH, dissolved oxygen (DO) and electrical conductivity (EC) were measured using electrodes in a flow cell, and water samples were collected with little exposure to the air. Sub-samples were collected from each bore for ferrous iron, total dissolved iron, nitrate, ammonium, DOC, cations and anions after filtering with a 0.45 μm filter. Samples were preserved in ice packs and transported to the lab for analysis.

Results and Discussion

Distribution of nitrate, ferrous iron and DOC in groundwater

The depth to water table, below ground level (bgl), ranged from 0.96-10.5m. There was a tendency for shallow water tables to occur near the coast and deeper water tables inland in both sampling times. Although the nitrate concentrations in the majority of the bores were below the ANZECC levels for long term environmental sustainability values as high as 14.4 mg of NO₃^--N/L were measured. During the September/October 2003 sampling period, only 25% of the bores showed NO₃^--N levels higher than 0.1 mg/L, with three bores above 5 mg/L. In contrast, 77% of the single bores yielded concentrations higher than 0.1 mg/L in January 2004 (Figure 2). There were five single bores which have NO₃^--N values above 5 mg/L. Elevated nitrate levels were mostly observed in the Airville-Homehill area. This applied to both sampling periods. Brodie et al (1984) and Weier (1999) also reported a similar spatial distribution but the magnitude of the concentrations they reported were higher.

Figure 2. Spatial distribution of NO₃-N^- (mg/L) in the lower Burdekin during January 2004

Figure 3. NO₃-N^- concentrations in mg/L at different depths

The vertical distribution of nitrate concentration revealed different trends between sampling periods. In the first sampling period not many bores showed higher nitrate levels at shallow depths (< 30 m) while six of the 28 sampled bores had values more than 5 mg NO₃-N^- /L in the second sampling (Figure 3). The observed differences in the vertical nitrate concentration profile with time could be related to fertiliser applications and subsequent irrigations. It is worth noting that different bores were sampled during the two periods. A steady decline in nitrate concentrations from the wet season to the dry season has been noted previously by Brodie et al. (1984). Elevated nitrate concentrations occurred mostly in bores located in areas where water table depths ranged between 2 and 6 m. Similar patterns have been observed with nitrate levels highest near the water table and decreasing to below detection with depth both at this site by Brodie et al. (1984) and at other sites by Trudell et al. (1986) and McLarin et al. (1999). With the exception of one shallow pipe located in Ayr, none of the nested bores had concentrations above 0.1 mg/L irrespective of the sampling time. The nested bores were mostly located along the coast. Nitrite concentrations were below detection or negligible (<0.1 mg/L) in all of the bore samples. Concentrations of NH₄⁺ reached 8.1 mg/L in the September/October sampling period while lower concentrations (maximum of 3.6 mg/L) were measured in bores sampled during January 2004. Its distribution showed no particular spatial or vertical trend, but nested bores showed higher values.

Figure 4. Distribution of ferrous iron (mg/L) in the Lower Burdekin groundwater
a) during September/October 2003 b) January 2004

The distribution of ferrous iron in the aquifer during the two sampling periods is shown in Figure 4 with values ranging from 0 to 361mg/L. The bores sampled during September/October 2003 had very high concentrations which may be related to the particular location of the bores. While we have only limited data at this stage, there is some evidence of spatial trends with high ferrous concentrations in the Kalamia Mill and Ayr areas. These results are consistent with the findings of Kelly (1974). Elevated ferrous levels were also observed in the Giru-Barratta area where concentrations of 3.02 - 15.42 mg/L were measured. Ferrous levels were below 2 mg/L in the Airville - Home Hill area.It is worth noting that elevated nitrate concentrations were observed in these areas. In general, elevated nitrate concentrations were not found in bores along the coast where elevated ferrous concentrations were common. As shown in Figure 5 ferrous iron concentrations were quite variable but tend to be higher in bores less than 20 metres deep, compared to deep bores (>55 metres deep) where the concentration ranged between 1.22 and 3.20 mg/L. The spatial and vertical trends in ferrous iron concentrations were consistent in both the sampling periods. Ferrous concentration in the nested bores ranged from 1.15 to 13.5 mg/L, and with the exception of one bore, there was no distinct vertical pattern. This could be influenced by the fractured nature of the aquifer materials and the prevailing geochemical conditions.

The DOC levels in the lower Burdekin aquifer were very high and variable both spatially and vertically. It ranged from 4 mg/L to 82 mg/L. One reason for these high levels could be the movement of sugar juices lost during harvest moving to deeper depths and entering the shallow groundwater systems. These high DOC concentrations in groundwater are also not that surprising when compared with the DOC concentration of 260 mg/L in the first runoff water from one of the sugar cane farms. A conservative estimation of Rayment in 2000 reported 0.13 tonnes of sucrose-C/ha/yr (0.33 tonnes of sucrose/ha/yr) loss to the soil/water environment during cane harvest. It is likely that the elevated levels of DOC in the lower Burdekin aquifer degrade with ferric iron as the terminal acceptor and results in the production of reduced soluble ferrous iron in anaerobic situations.

Figure 5. Ferrous concentrations at different depths during September/October 2003 and in January 2004.

Other geochemical conditions

In most bores, the DO concentrations were below 1 mg/L in both sampling periods. Although the concentration varied considerably at shallow depths it decreased with the water table depth. In the nested bores DO concentrations did not show the decreasing trend with depth as reported by others (Kelly, 1997). They were however, all below 0.7 mg/L meaning that anoxic conditions prevailed in the groundwater.

Eh varied between -120 and 235 mV, with little evidence of trends with increasing depth. In the bores sampled, DO varied from 0.1 to 6 mg/L between an Eh of 0 and 235 mV but was <1 mg/L below 0 mV. Elevated NO₃^--N concentrations occurred in groundwater at Eh values between 37 and 235mV, and NO₃^--N was largely undetectable with Eh less than 0. In contrast to the NO₃^--N profile, elevated ferrous concentrations (>4 mg/L) occurred in groundwater with Eh below 0 mV and a DO concentration lower than 1 mg/L.

Denitrification potential

Nitrate is a redox-sensitive species and it would be expected that loss of nitrate due to reduction or denitrification will be accompanied by changes in other redox species, changes in geochemical conditions, and the production of reduced forms of nitrogen (Puckett and Cowdery 2002). As such, an evaluation of the dominant species of the major redox-active elements including oxygen, iron, manganese, sulphur, carbon, and nitrogen together with the geochemical conditions can provide clues to the potential of the aquifer for denitrification. As discussed earlier the DO concentration varied widely however, only 2 of the groundwater samples had values greater than 2.0 mg/L, indicating favourable conditions. DO levels of less than 2.0 mg/L was reported to be favourable for denitrification (Korom 1992). Furthermore, the Eh values from -120 to + 235mV tend to support the existence of favourable reduced conditions for denitrification in the aquifer.

A plot of nitrate versus ferrous iron or ammonium for samples taken shows the relationship between these parameters; the presence of one is usually marked by the absence of the other (Figure 6). The observed negative correlations between nitrate and iron or ammonium suggest that the nitrate in the groundwater is being consumed. This could be one of the reasons for little change observed in nitrate concentration in 90% of wells monitored in the Burdekin region (Thorburn et al. 2003). It appears that two processes are involved, one relating to ferrous and the other to high ammonium levels. As discussed earlier bacteria use iron in its reduced ferrous form as an electron donor to reduce nitrate in the absence of dissolved oxygen.

Figure 6. NO₃^--N concentration versus ferrous iron concentration

Based on the stoichiometry of denitrification in equation (2), at the median NO₃-N concentration of 0.51 mg/L at least 10.2 mg/L of ferrous iron are required. It appears that the ferrous iron available is not enough for denitrification across the entire area of the lower Burdekin however, the higher amounts of ferrous iron found in the Kalamia mill-Ayr area indicates that this area has good potential for denitrification. A map showing potential areas for denitrification in the lower Burdekin was prepared with reference to the ferrous iron concentrations used by the Minnesota Pollution Control Agency (1999) to define nitrate sensitive areas. Figure 7 shows areas of low and high potential for denitrification at shallow (a) and deep depths (b). The map suggests that the shallow groundwater has potential for denitrification in most of the areas compared to the deep groundwater.

Figure 7. Map showing areas of low and high potential for denitrification in the lower Burdekin

The existence of low nitrate concentrations in bores with high levels of ammonium, and the reduced environment in the groundwater suggest that Dissimilatory Nitrate Reduction (DNRA) could be occurring (Figure 8). Ammonium (NH₄⁺) is the end product of DNRA, which is a process that can temporarily remove nitrate as long as NH₄⁺ does not come in contact with aerobic environments (Korom 1992; Tesoriero et al. 2000). DNRA appears more in salt marshes and in strongly anoxic marine sediments (Postma et al. 1991). Marine deposits are present within the Burdekin aquifer which seems to support the existence of dissimilatory denitrification processes.

Figure 8 NO₃^--N concentration versus NH₄⁺-N concentration

There was no consistent pattern of high nitrate concentrations associated with low DOC concentrations. Considering DOC as the most preferred electron donor and given the stoichiometry of 1:1.25 (molar basis) between nitrate and DOC required for denitrification processes (Korom, 1992), the range of DOC (4 to 82 mg/L ) measured in the groundwater is enough to denitrify a large amount of nitrate at a depth of < 30 m. It means that we should not expect nitrate at this depth. The fact that we do suggest that either biological constraints exist or that the denitrification rate is slow. While beneficial on the point of view of promoting denitrification, the high DOC loading to the aquifer increases the weathering rates of iron and manganese oxides.

Conclusion

Given the spatial variation in nitrate concentrations and the fact that the nitrate input is not constant with time, it is difficult to determine the extent of nitrate reduction which has taken place within the aquifer. However, the measured values of DO, Eh and pH which can be used to characterise the geochemical conditions of the aquifer, are near the upper threshold levels given in the literature to support the denitrification process. Also, the negative correlation between nitrate and the levels of ferrous iron indicates that there has been at least some denitrification. The presence of higher NH₄⁺ in places where the nitrate is lower and, vice versa suggests the occurrence of DNRA. Furthermore, higher levels of ferrous iron found both in Ayr-Kalamia Mill areas and near the coast indicate that these areas have potential for removing nitrate from the groundwater. High levels of DOC with a wide spatial distribution indicates that the whole area has the potential for denitrification. More studies are required in order to quantify the amount of nitrate loss from the aquifer due to denitrification and to delineate nitrate insensitive or potential denitrification depths.

The map presented in this paper is a first step towards identifying those areas of the lower Burdekin with most potential for denitrification. Although generalized, it is a valuable start in helping determine the fate of nitrate entering the groundwater. This information is important in helping to understand the complexities of the floodplain and in enabling resource managers to better target their management practices.

Acknowledgement

The authors thank Ray Mcgowan, Bob Mooney and Steve Porter from NRM&E for their assistance in the field. This work was partly supported by CRC IF.

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