HIGH NITRATE CONCENTRATIONS IN THE DOUGLAS RIVER AND THEIR IMPACT ON DALY RIVER WATER QUALITY

Transcription

1 HIGH NITRATE CONCENTRATIONS IN THE DOUGLAS RIVER AND THEIR IMPACT ON DALY RIVER WATER QUALITY Julia Schult and Rodney Metcalfe 26 Report 9/26D Aquatic Health Unit Environment and Heritage Division

2 Front Cover: Overhanging vegetation on the South bank of the Douglas River, 8 km upstream of Tipperary Crossing (R. Metcalfe)

3 Contents List of Tables...ii List of Figures...iii SUMMARY INTRODUCTION OBJECTIVES METHODS DOUGLAS RIVER WATER QUALITY AND FLOW MIXING AT THE CONFLUENCE OF THE DOUGLAS AND DALY RIVERS DALY RIVER WATER QUALITY DOWNSTREAM OF THE CONFLUENCE RESULTS WATER QUALITY DOUGLAS RIVER FLOW COMPARISON OF HISTORICAL AND CURRENT NITRATE LEVELS NUTRIENT LOADS MIXING OF THE DOUGLAS AND DALY RIVERS DALY RIVER WATER QUALITY DOWNSTREAM OF THE CONFLUENCE Integrated Samples (NO 2, NO 3, FRP) Integrated samples (Chl-a) Grab samples (NO 3, FRP and Chlorophyll a) Euphotic depth DISCUSSION REFERENCES...17 APPENDIX 1: IONIC COMPOSITION OF THE DOUGLAS RIVER, HAYES CREEK AND THE DALY RIVER...18 APPENDIX 2: IN SITU WATER QUALITY MEASUREMENTS FOR SEVEN TRANSECTS ACROSS THE DOUGLAS AND DALY RIVERS...19 High NO 3 in the Douglas River i

4 List of Tables TABLE 1. DRY SEASON FLOWS IN LATE OCTOBER 24 AT SIX GAUGING SITES (HAYES CREEK, DOUGLAS RIVER AND DALY RIVER)...8 TABLE 2. NUTRIENT LOADS (KG/DAY OF N OR P) IN THE DOUGLAS AND DALY RIVERS IN OCTOBER TABLE 3. MEANS AND RANGES OF NO 3 AND FRP CONCENTRATIONS FOR DOUGLAS AND DALY RIVER TRANSECTS (DEPTH INTEGRATED SAMPLES) TABLE 4. MEAN CHLOROPHYLL A CONCENTRATIONS FOR DOUGLAS AND DALY RIVER TRANSECTS (DEPTH INTEGRATED SAMPLES) TABLE 5. CONDUCTIVITY, NO 3, FRP AND CHL-A AT THE SURFACE AND BOTTOM OF THE WATER COLUMN AT TWO LOCATIONS WITH LARGE VERTICAL CONDUCTIVITY DIFFERENCE. THE SURFACE WATERS OF THE DOUGLAS RIVER AT ITS CONFLUENCE WITH THE DALY IS DALY RIVER WATER. THE BOTTOM WATERS HOWEVER ORIGINATE FROM THE DOUGLAS RIVER...14 TABLE 6. EUPHOTIC DEPTH AND CHANNEL DEPTH IN THE DOUGLAS AND DALY RIVERS...14 TABLE 7. HISTORICAL NITRATE DATA FROM SOME BORES IN THE DOUGLAS REGION SHOWING ELEVATED NITRATE LEVELS...16 High NO 3 in the Douglas River ii

5 List of Figures FIGURE 1. MAP OF DOUGLAS RIVER FLOW GAUGING AND WATER QUALITY SITES AND AQUIFERS IN THE REGION....3 FIGURE 2. DIAGRAM OF DALY RIVER TRANSECTS...4 FIGURE 3. DUROV DIAGRAM OF IONIC COMPOSITION, SALINITY (TOTAL DISSOLVED SOLIDS) AND PH OF THE DOUGLAS RIVER, HAYES CREEK AND THE DALY RIVER (N=7)...6 FIGURE 4. WATER QUALITY MEASUREMENTS AT DOUGLAS RIVER SITES (CLOSED CIRCLES). HAYES CREEK (SITE 4) AND DALY RIVER (SITE 6) ARE REPRESENTED BY OPEN CIRCLES...7 FIGURE 5. FLOW AND LONGITUDINAL CONDUCTIVITY AND NITRATE GRADIENTS IN THE DOUGLAS RIVER BETWEEN 1999 AND 24. OPEN CIRCLES REPRESENT MEASUREMENTS FROM MIDDLE CREEK (23) AND HAYES CREEK (24). HISTORICAL DATA FROM TICKELL ET AL. 22, TOWNSEND 22 AND NRETA UNPUBLISHED DATA....9 FIGURE 6. CHANGES IN NITRATE CONCENTRATION BETWEEN 1999 AND 24 AT TWO DOUGLAS RIVER SITES, THE WEIR (SITE 3) AND CRYSTAL FALLS (SITE 5)...1 FIGURE 7. CONTOUR GRAPH OF CONDUCTIVITY FOR SEVEN CROSS SECTIONS OF THE DOUGLAS AND DALY RIVERS. VALUES ARE IN µs/cm. DOUGLAS RIVER WATER OF LOWER CONDUCTIVITY (BLUE) ENTERS THE DALY RIVER (HIGHER CONDUCTIVITY, GREEN) AND MIXES GRADUALLY UNTIL THE RIVER IS FULLY MIXED 15 M DOWNSTREAM OF THE CONFLUENCE FIGURE 8. RELATIONSHIP BETWEEN CONDUCTIVITY (AT BOTTOM OF WATER COLUMN) AND NO 3 CONCENTRATION. CONCENTRATIONS FROM DEPTH INTEGRATED SAMPLES High NO 3 in the Douglas River iii

6 Summary Dry season water quality surveys of the Douglas River between 1999 and 23 have shown that nitrate concentrations in the lower reaches are high (Townsend 22, NRETA unpublished data).. These high nitrate concentrations persisted during 24. The rise in nitrate concentration coincides with groundwater input from the Tindal Limestone aquifer to the Douglas River. The concentration of nitrate has been variable between years but has consistently been 1 to 2 times higher than in the Daly River and other creeks in the area. At the confluence of the Douglas and Daly Rivers, the nitrate-rich Douglas River flowed under the Daly River in November 24 due to its higher density caused by cooler water temperature. Mixing of the two rivers occurred gradually and was complete 15 m downstream of the confluence. At other times in the dry season, this mixing phenomenon may be different, depending on the relative densities of the two rivers. The addition of nitrate did not result in elevated phytoplankton concentrations and did not seem to have a major effect on the rivers ecology. High NO 3 in the Douglas River 1

7 1 Introduction The availability of nutrients is an important factor in determining phytoplankton biomass in streams. High levels of nutrients, especially soluble forms of nitrogen and phosphorus that are immediately available for algal uptake, can contribute to high concentrations of algae that may constitute an algal bloom. Such blooms have a detrimental effect on water quality and other aquatic biota. Water quality studies in the Douglas-Daly region detected elevated levels of nitrate in the Douglas River during the dry season between 1999 and 23 (Townsend 22, NRETA unpublished data). The nitrate concentration in the Douglas River increased markedly in an area near the Oolloo Road bridge, approximately 28 km upstream of the river mouth. In this region groundwater from the Tindal Limestone aquifer enters the Douglas River (Jolly et al. 2, Tickell et al. 22). The source of the elevated nitrate concentrations is currently unknown. Possible explanations put forward by stakeholders include naturally high nitrate levels in the Tindal Limestone aquifer, pollution from septic tank leakage, nutrients originating from animal faeces such as bat droppings, seepage of nitrogen into the groundwater from current or past application of fertilisers or growing of nitrogen-fixing legumes in agriculture. The nutrient enrichment of the Douglas River has the potential to influence the water quality of the Daly River. Although Townsend (22) did not detect an impact of these high nitrate levels on the water quality of the Daly River 62 km downstream, impacts may occur closer to the Douglas River s confluence with the Daly River. 2 Objectives The objectives of this study were to investigate whether high nitrate concentrations were present in the Douglas River in the 24 dry season measure dry season flows of the Douglas River determine nutrient loads of the Douglas and Daly Rivers determine how the nitrate-rich Douglas River mixes with the Daly River examine if this nutrient enrichment has an effect on phytoplankton in the Daly River. High NO 3 in the Douglas River 2

8 3 Methods 3.1 Douglas River water quality and flow To investigate nutrients in the Douglas River, water samples were taken at six locations on October 24 (Figure 1). Sites 1 (G ) and 2 (G ) were located on the Douglas River, 4 km and 2 km upstream of the Oolloo Road bridge, Site 3 (G ) at a weir near the Douglas-Daly Caravan Park and Site 5 (G ) at Crystal Falls, near the mouth of the Douglas River. Site 4 (G ) was located on Hayes Creek just upstream of its confluence with the Douglas River, and Site 6 (G814513) on the Daly River, upstream of the Douglas-Daly confluence Kilometers H ay es C re e k ± Tipperary Crossing Do ug s la ve Ri r Mid dle C re e k O ol lo o Rd 6 Daly River 24 Gauging Sites Jinduckin Formation Oolloo Dolostone Tindal Limestone Figure 1. Map of Douglas River flow gauging and water quality sites and aquifers in the region. At each site, conductivity, ph, temperature and dissolved oxygen content were measured with a Hydrolab multi-parameter probe. Water samples were taken for analysis of major ions, nitrate (NO3), nitrite (NO2), total Kjeldahl nitrogen (TKN), filterable reactive phosphorus (FRP) and total phosphorus (TP). Samples were stored High NO3 in the Douglas River 3

9 on ice and frozen on return to the laboratory until just before the chemical analyses. Flow was measured by standard hydrographic techniques, using a propeller-type current meter. Nutrient loads were determined from the product of flow and nutrient concentration. 3.2 Mixing at the confluence of the Douglas and Daly Rivers Upstream of the confluence the Daly River has a uniform conductivity of 65 µs/cm while conductivity at Site 5 in the Douglas River was about 512 µs/cm. This conductivity difference of approximately 9 µs/cm was used as a marker to trace the flow direction and degree of mixing of the two rivers. Profiles of physical characteristics were measured at several points across seven transects in the Douglas and Daly Rivers on 28 October 24 and 2 November 24 (Figure 2). At each point conductivity, temperature, ph and dissolved oxygen were measured at 1 m intervals along the water column with a Hydrolab multi-parameter probe to obtain a three-dimensional picture of the pathway the Douglas River takes downstream of the confluence. Douglas River Daly River m 1m 25 m 5m 15 m Figure 2. Diagram of Daly River transects 3.3 Daly River water quality downstream of the confluence Depth integrated water samples were collected at each point along the transects using a 5 m sampling tube that was lowered into the water to just above the river bed. A valve prevented backflow of water and the content of the tube was then emptied into a bucket from which sub-samples were taken for analyses of NO 3, NO 2, FRP and chlorophyll a. Photosynthetically available radiation (PAR) was measured near the midpoint of each transect using a Licor light sensor. High NO 3 in the Douglas River 4

10 In two locations, where conductivity indicated a layering of waters from the Douglas and Daly Rivers, separate grab samples were taken from the top and bottom of the water column to compare the results with those from integrated samples. All chemical analyses were carried out according to APHA standard methods (APHA 1998) by the Northern Territory Environmental Laboratories (NTEL) with the exception of chlorophyll a, which was analysed by the water chemistry laboratory of the then Department of Business, Industry and Resource Development (DBIRD). Where concentrations were below detection limit, half the detection limit was used in calculations of means and statistical analyses. 4 Results 4.1 Water Quality Ionic composition The Durov diagram (Figure 3) summarises the ionic composition, salinity and ph of water samples. The triangular sections show the relative dominance of different anions (Mg 2+, Ca 2+, Na +, K + ) and cations (SO 2-4, HCO - 3, Cl - ). The more the composition is dominated by a particular ion, the closer it is to the corresponding apex of the triangle. The concentration of TDS (salinity) is shown on the right of the diagram on a logarithmic scale while the ph is indicated in the bottom section. The central square of the diagram represents a synthesis of all of the above components. Samples with similar overall water quality will be close together, while those with differing qualities will be further apart. The Douglas River is dominated by magnesium, calcium and bicarbonate ions. However, Site 1 on the upper Douglas River has a larger component of sodium, lower TDS and lower ph than all other sites. Water of the upper reaches of the Douglas River is of low conductance, low hardness, low alkalinity and neutral ph. It is dominated by sodium, calcium and bicarbonate. Further downstream the anions are dominated even more strongly by bicarbonate while the cation composition is more calcium and High NO 3 in the Douglas River 5

11 magnesium dominated. Hayes Creek has a similar ionic composition to the lower Douglas River, indicating that it is fed from the same aquifer. Full details of the ionic composition of samples are given in Appendix 1. SO 4 Ca Cl HCO 3 Total Dissolved Solids (mg/l) Mg Na+K ph Hayes Creek Daly River Douglas River (Site 1) Douglas River (Sites 2-5) 9. Figure 3. Durov Diagram of ionic composition, salinity (total dissolved solids) and ph of the Douglas River, Hayes Creek and the Daly River (n=7). Physico-chemical parameters Physical and chemical water quality measurements are shown in Figure 4. The water quality of the Douglas River changed markedly between Site 1 and Site 3. Conductivity increased almost 1-fold from 44 µs/cm to 424 µs/cm, while temperature, ph, dissolved oxygen and turbidity fell considerably. There was a steep rise in nitrate concentrations in the Douglas River between Site 1 and Site 3. While nitrate was below the detection limit of 1 µg/l at Site 1 (Note: all nutrient concentrations are given as µg/l of N or P), the concentration reached 56 µg/l at Site 3, only 5 km further downstream. Nitrate concentrations in both Hayes Creek (Site 4) and the Daly River (Site 6) were very low. High NO 3 in the Douglas River 6

12 Temperature ph DO (%) FRP (μg/l) NO 3 (μg/l) NO 2 (μg/l) Conductivity (μs/cm) TP (μg/l) Turbidity (NTU) TKN (μg/l) Sample Site Sample Site Figure 4. Water quality measurements at Douglas River sites (closed circles). Hayes Creek (Site 4) and Daly River (Site 6) are represented by open circles. Levels of other nutrients were low at all sample sites. Filterable reactive phosphorus ranged from 5 to 8 µg/l, total phosphorus from 5 to 15 µg/l and Total Kjeldahl Nitrogen from 11 µg/l in the Daly River to 42 µg/l at Site 2 on the Douglas River. Nitrite concentrations were below the detection limit of 1 µg/l at three sites and never exceeded 2 µg/l. High NO 3 in the Douglas River 7

13 4.2 Douglas River Flow The Douglas River gains water where it intersects the Tindal Limestone and Oolloo Dolostone aquifers of the Daly Basin (see Figure 1). Douglas River flows were measured by Tickell et al. (22) in 22 between the Douglas Hot Springs and Crystal Falls (Site 5). The river gained approximately 2.5 m 3 s -1 from the Tindal aquifer between the Hot Springs and the Douglas Daly Research Farm (Tipperary Crossing). The Oolloo aquifer contributed a further.7 m 3 s -1 between Tipperary Crossing and Site 5 at Crystal Falls. In 24 flow in the Douglas River increased by.52 m 3 s -1 between Site 1 and Site 3 (Table 1). The river gained another 3 m 3 s -1 between Site 3 and Site 5 to a total flow of approximately 4.2 m 3 s -1 at Crystal Falls (Site 5), just upstream of the Douglas River mouth. This latter gain is made up of water from both the Tindal and Oolloo aquifers. Hayes Creek contributed only.12 m 3 s -1 to the Douglas River. The Daly River flow was 24.8 m 3 s -1 upstream of the Douglas confluence. Table 1. Dry season flows in late October 24 at six gauging sites (Hayes Creek, Douglas River and Daly River). Site No. Site Date Discharge (m 3 s -1 ) 1 Douglas 4 km u/s Oolloo Rd bridge 29/1/ Douglas 2 km u/s Oolloo Rd bridge 27/1/ Douglas V-Notch Weir 27/1/ Hayes Ck 26/1/ Douglas Crystal Falls 26/1/ ( )* 6 Daly u/s confluence 26/1/24 25 * very slow flow at Crystal Falls led to increased gauging error, therefore a range is given here 4.3 Comparison of historical and current nitrate levels The Douglas River near the Hot Springs has low conductivity and is likely to originate from a non-carbonate rock source outside the Daly River Basin. Water from the Tindal aquifer is characterised by conductivities of 5-6 µs/cm. Between Site 1 and Site 3, where inflow from this aquifer occurs, the conductivity of the Douglas River rises sharply (Figure 4). High NO 3 in the Douglas River 8

14 Flow (cumecs) Conductivity (μs/cm) Conductivity (μs/cm) 4 2 Flow (cumecs) Nitrate (μg/l_n) Nitrate (μg/l_n) Distance from Butterfly Gorge (km) Distance from Butterfly Gorge (km) Distance from Butterfly Gorge (km) Conductivity (μs/cm) Conductivity (μs/cm) 4 2 Conductivity (μs/cm) Nitrate (μg/l_n) Nitrate (μg/l_n) Nitrate (μg/l_n) Distance from Butterfly Gorge (km) Distance from Butterfly Gorge (km) Distance from Butterfly Gorge (km) Figure 5. Flow and longitudinal conductivity and nitrate gradients in the Douglas River between 1999 and 24. Open circles represent measurements from Middle Creek (23) and Hayes Creek (24). Historical data from Tickell et al. 22, Townsend 22 and NRETA unpublished data. High NO 3 in the Douglas River 9

15 Figure 5 shows longitudinal patterns in conductivity and nitrate concentrations between 1999 and 24. Nitrate concentrations in the river rise in the same area as conductivity, suggesting that nitrate enters with groundwater from the Tindal aquifer. However, neither Hayes Creek nor Middle Creek, both fed by the Tindal aquifer, show elevated nitrate concentrations. Nitrate concentrations were variable over the years but have consistently been elevated. A comparison of nitrate concentrations over the five year period shows a downward trend in the data over time (Figure 6). In 21 nitrate levels in the Douglas River reached a maximum of 125 µg/l near the Douglas-Daly confluence. Nitrate concentrations at Sites 3 and 5 were considerably lower in 23 and 24 compared to previous years. 14 Nitrate (μg/l_n) Site 3 Site Figure 6. Changes in nitrate concentration between 1999 and 24 at two Douglas River sites, the weir (Site 3) and Crystal Falls (Site 5). There is some suggestion that nitrate levels in the Douglas River fall again with the inflow from the Oolloo aquifer between Tipperary Crossing and Site 5 (Figure 5, 24). In 24, the nitrate concentration was higher at Tipperary Crossing (65 µg/l) than at Site 5 (46 µg/l). Additional sampling is needed to confirm this trend. 4.4 Nutrient loads Loads of TKN, TP, FRP and NO 2 were higher in the Daly River than in the Douglas River, due to its considerably larger flow (Table 2). The NO 3 load of the Douglas River at Sites 3 and 5, however, exceeded that of the Daly River. The Douglas River carried High NO 3 in the Douglas River 1

16 approximately 17 kg of nitrate nitrogen per day while the load of the Daly River was only 1.1 kg per day. Table 2. Nutrient loads (kg/day of N or P) in the Douglas and Daly Rivers in October 24 No. Stream Site NO3 NO2 FRP TP TKN 1 Douglas 4 km u/s Oolloo Rd bridge Douglas 2 km u/s Oolloo Rd bridge Douglas V-Notch Weir Hayes Ck Hayes Creek Douglas Crystal Falls Daly Daly River u/s confluence Mixing of the Douglas and Daly Rivers Profiles from transects showed that the Douglas River flow followed a path from the confluence to the bottom of the Daly River (Figure 7). It was directed underneath waters of the Daly River despite its lower conductivity. This layering was determined by the Douglas River s lower temperature of C compared to C in the Daly River upstream of the confluence. The colder water of the Douglas River followed the river s thalweg (deepest point). Mixing occurred gradually and was complete 15 m downstream of the confluence where the conductivity was again equal at all points of the cross-section. The full set of water quality data for all crosssectional profiles, including temperatures, ph and dissolved oxygen content is provided in Appendix Daly River water quality downstream of the confluence Integrated Samples (NO 2, NO 3, FRP) The mean nitrate concentration in the Douglas River near its mouth was 9.2 µg/l. Upstream of the Douglas-Daly confluence nitrate concentrations were below the detection limit of 1 µg/l in all Daly River integrated samples. Mean NO 3 in the Daly River rose downstream of the Douglas-Daly confluence and subsequently decreased again further downstream (Table 3). Mean filterable reactive phosphorus (FRP) concentrations were slightly lower in the Douglas River than in any of the Daly River transects, with 11 µg/l in the Douglas and 12 to 13 µg/l in the Daly River. NO 2 was undetectable (<1 µg/l) in all but six samples with concentrations of 1 µg/l. High NO 3 in the Douglas River 11

17 Douglas River Daly River m 1m 25 m 5m m Figure 7. Contour graph of conductivity for seven cross sections of the Douglas and Daly Rivers. Values are in µs/cm. Douglas River water of lower conductivity (blue) enters the Daly River (higher conductivity, green) and mixes gradually until the river is fully mixed 15 m downstream of the confluence. High NO 3 in the Douglas River 12

18 Table 3. Means and ranges of NO 3 and FRP concentrations for Douglas and Daly River transects (depth integrated samples). NO 3 (µg/l) FRP (µg/l) Site n Mean Range Mean Range Douglas Daly u/s 5 <1 < Daly < Daly < Daly Daly < Daly < The NO 3 concentration was significantly correlated to conductivity at the bottom of the water column, i.e. to the presence and degree of dilution of water from the Douglas River (Figure 8, r 2 =.47, p=<.1) NO 3 _N (μg/l) Conductivity difference to Daly u/s Figure 8. Relationship between conductivity (at bottom of water column) and NO 3 concentration. Concentrations from depth integrated samples Integrated samples (Chl-a) Chlorophyll a concentrations were low in all samples (<1.5 µg/l) with no evident increase downstream of the Douglas-Daly confluence (Table 4). Chlorophyll concentrations were not correlated to NO 3 or FRP concentrations. Table 4. Mean chlorophyll a concentrations for Douglas and Daly River transects (depth integrated samples). Mean Chl-a Range n Site (mg/l) Douglas Daly upstream of confluence Daly Daly Daly Daly Daly High NO 3 in the Douglas River 13

19 4.6.3 Grab samples (NO 3, FRP and Chlorophyll a) The four grab samples from the Douglas River mouth and the Daly River near the confluence show that nitrate concentrations varied markedly between the top and bottom of the water column, as expected in areas of differing conductivity where mixing had not yet taken place (Table 5). FRP and chlorophyll a showed no relationship with conductivity. Table 5. Conductivity, NO 3, FRP and Chl-a at the surface and bottom of the water column at two locations with large vertical conductivity difference. The surface waters of the Douglas River at its confluence with the Daly is Daly River water. The bottom waters however originate from the Douglas River. Site Depth (m) Cond. (µs/cm) NO 3 (µg/l_n) FRP (µg/l) Chl-a (µg/l) Douglas at confl. (surface) Douglas at confl. (bottom) Daly at confl. (surface).1 65 < Daly at confl (bottom) Euphotic depth The euphotic depth ranged from 7.7 m to 1.7 m and exceeded the depth of the water column at all sample sites (Table 6). A reduction in euphotic depth occurred 1 m downstream of the confluence and it increased again further downstream. Table 6. Euphotic depth and channel depth in the Douglas and Daly Rivers. Site Euphotic Depth (m) Maximum channel depth (m) Daly upstream Douglas mouth Daly at confluence m m m m Not measured 3.2 High NO 3 in the Douglas River 14

20 5 Discussion Historical and 24 data show that nitrate concentrations in the Douglas River rise markedly in the region of the Oolloo Road bridge. The concentrations of NO 3 in the Douglas River measured in 24 were elevated, but substantially lower than those reported by Townsend (22) in 1999 and 21. Townsend (22) found concentrations of up to 125 µg/l while the highest value in 24 was 56 µg/l. However this still translated into a NO 3 load in the Douglas River 16 times higher than that of the Daly River. Loads of all other nutrients were higher in the Daly River, in accordance with its much larger flow volume. The rise in NO 3 in the Douglas River coincides with a rise in conductivity, an indicator for inflow from the Tindal Limestone aquifer which has a conductivity of 5 6 µs/cm (Tickell et al. 22). Historical data of groundwater quality from bores in the Douglas/Daly region show that groundwater nitrate concentrations have risen considerably in three bores between the early 198s and early 199s (Table 7). While most bores had NO 3 levels below the detection limit of.22 mg/l_n throughout this period, the concentrations in some bores rose from very low levels in the early 198s to up to 6.4 mg/l_n in the early 199s. This rise in nitrate levels coincides with increased agricultural development in the region in the 198s and early 199s. It should be noted though, that the number of bores with elevated nitrate is a small proportion of all bores in the area, and that not all bores have been sampled with the same frequency. Furthermore, that the bores with high nitrate concentrations are not located in the one area. It s possible these high nitrate concentrations result from local contamination of the groundwater. A thorough investigation of groundwater nitrate concentrations is needed to help interpret the significance of the high nitrate concentrations measured in bores some time ago. The source of high nitrate concentrations in the Douglas River is discussed in a National Action Plan for Salinity and Water Quality report on nutrients in the Douglas-Daly Region (Schult et al. in preparation). Hayes Creek and Middle Creek both also receive water from the Tindal Limestone aquifer, yet they do not have high nitrate concentrations. High NO 3 in the Douglas River 15

21 Table 7. Historical nitrate data from some bores in the Douglas region showing elevated nitrate levels. RN 2628 RN819 RN21153 NO 3 _N NO 3 _N NO 3 _N Date (mg/l) Date (mg/l) Date (mg/l) Jun Jul May-82 <.22 Sep May May May May May Jun Dec May May May May Nov The high nitrate concentrations in the Douglas River did not appear to have an effect on phytoplankton in the Daly River. After reaching the Daly River, the Douglas River flowed beneath the Daly River due to its cooler temperature, and its nitrate-rich flow was diluted by the Daly River. This mixing pattern was determined by the lower temperature of the Douglas River and could change over the course of a year. At equal temperatures, water of lower conductivity will form a layer on top of higher conductivity water due to its lower density. If the temperature difference between the Douglas and Daly Rivers was smaller, the nitrate rich Douglas River water may not flow underneath the Daly River but form a layer on top of it or conceivably enter the Daly River mid-profile. Although nitrate concentrations in the Daly River increased downstream of the confluence, this did not translate into increased phytoplankton concentrations in the 1.5 km study reach downstream of the confluence. Light limitation does not appear to be of importance, since the water was clear and the euphotic zone extended far beyond the river bed at all sample sites. The average velocity at the Daly River gauging site, upstream of the confluence, was.19 m/s. At this velocity, the time taken for water to flow from the Douglas-Daly confluence to the site of complete mixing 15 m downstream is approximately 13 minutes. This time is not sufficient for a noticeable reaction by phytoplankton to the changed nutrient availability in the Daly River and if the addition of nitrate had an effect on phytoplankton biomass, it would not be detectable until much further downstream. However, chlorophyll concentrations measured by Townsend (22) at High NO 3 in the Douglas River 16

22 Beeboom Crossing, 62 km downstream of the Douglas-Daly confluence, were very low (< 1 µg/l) and chlorophyll concentrations at the mouth of the Douglas River were only marginally higher than in the Daly River. This indicates that nitrate is not a limiting factor for the growth of phytoplankton in the Daly River. Although phytoplankton does not appear to be affected by higher nitrate availability this study did not address the effects on benthic algae. It is possible that the elevated nitrate concentrations have an effect on benthic algae near the confluence that are continuously exposed to higher nitrate concentrations. It is likely that phosphorus, not nitrogen, is the nutrient limiting phytoplankton biomass in the Douglas and Daly Rivers since nitrate addition did not result in elevated phytoplankton concentrations and did not have a major effect on the rivers ecology. This assumes however that other factors, notably flow and the constant carriage of algae downstream, are not limiting algal biomass. 6 References APHA (1998). Standard Methods for the Examination of Water and Wastewater. 2 th Edition. (Eds: Clesceri, L.S., Greenberg, A.E., Eaton A.D.). American Public Health Association, Washington DC. Jolly, P., George, D., Jolly, I., Spiers, Z. (2). Analysis of Groundwater Fed Flows for the Flora, Katherine, Douglas and Daly Rivers. Report 36/2D. Department of Lands, Planning and Environment, Darwin, NT. Townsend, S.A. (Ed.) (22). Periphyton and Phytoplankton Response to Reduced Dry Season Flows in the Daly River. Department of Infrastructure, Planning and Environment, Darwin, NT. Tickell, S.J. (22). A Survey of Springs along the Daly River. Report 6/22. Department of Infrastructure, Planning and Environment, Darwin, NT. Tickell, S.J., Cruikshank, S., Kerle, E. and Willis, G. (22). Stream Baseflows in the Daly Basin. Report 36/22. Department of Infrastructure, Planning and Environment, Darwin, NT. High NO 3 in the Douglas River 17

23 Appendix 1: Ionic composition of the Douglas River, Hayes Creek and the Daly River PH, conductivity, hardness and alkalinity of the Douglas River, Hayes Creek and the Daly River. Total Hardness (CaCO3) Total Alk (CaCO3) SiO2 NaCl Site ph Cond TDS a b Concentrations of cations (mg/l) in the Douglas River, Hayes Creek and Daly River. Site Na + K + Ca 2+ Mg 2+ Fe a b <.1 Concentrations of anions (mg/l) in the Douglas River, Hayes Creek and Daly River. Site Cl - 2- SO 4 - NO 3 - HCO 3 2- CO 3 OH - F <1 24 < < < < a 4 6 < b 4 6 < <

24 Appendix 2: In Situ water quality measurements for seven transects across the Douglas and Daly Rivers. River Dist. from confl. Date Wid th Dist. WERB Total Depth Depth Temp ph DO DO(%) Cond Turb* Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Daly 1 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Douglas 25 m u/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/

25 River Dist. from confl. Date Wid th Dist. WERB Total Depth Depth Temp ph DO DO(%) Cond Turb* Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 2/11/ Daly m d/s 2/11/ Daly m d/s 2/11/ Daly m d/s 2/11/ Daly m d/s 2/11/ Daly m d/s 2/11/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/

26 River Dist. from confl. Date Wid th Dist. WERB Total Depth Depth Temp ph DO DO(%) Cond Turb* Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 1 m d/s 28/1/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 25 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/

27 River Dist. from confl. Date Wid th Dist. WERB Total Depth Depth Temp ph DO DO(%) Cond Turb* Daly 5 m d/s 2/11/ Daly 5 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 1 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ Daly 15 m d/s 2/11/ * Turbidity as determined from integrated water samples