Nitrogen losses from a domestic septic tank system on the Darling Plateau in Western Australia

Pergamon

0043-1354 (95)00023-2

War. Res. Vol. 29, No. 9, pp. 2055-2058, 1995 Copyright © 1995ElsevierScienceLid Printed in Great Britain.All rights reserved 0043-1354/95 $9.50 + 0.00

NITROGEN LOSSES FROM A DOMESTIC SEPTIC TANK SYSTEM ON THE DARLING PLATEAU IN WESTERN AUSTRALIA R O B E R T G. GERRITSE*, J O H N A. A D E N E Y and J A N H O S K I N G Division of Water Resources, CSIRO, Private Bag, P.O. Wembley, W.A. 6014, Australia

(First received May 1994; accepted in revisedform January 1995) Abstract--Movement in groundwater of nitrogen, leaching from a domestic septic tank system, was studied in an area adjacent to a stream on the Darling Plateau in the Shire of Mundaring near Perth in Western Australia. Losses of nitrogen in soil were determined by adding bromide to the septic tank and monitoring the decrease in the ratio of nitrogen to bromide in groundwater, downgradient of the leach drain of the septic tank. At least 80% of inorganic nitrogen leaching into the soil appears to be lost within a distance of 10 m from the leach drain. Published work on changes in 'SN/~4Nisotope ratios of nitrate in sandy aquifers, affected by domestic septic tank systems, seems to indicate that losses of nitrogen are less at higher latitudes.

Key words--nitrate, soil, groundwater, denitrification, septic tank

INTRODUCTION Residential developraent on the Darling Plateau near Perth in Western Australia is not sewered. Waste is processed in septic tanks and the resulting effluent leaches into the subsoil from one or more drains. Accumulated solids are pumped from the septic tanks at about 5 year intervals and processed to compost in a treatraent plant on the Swan Costal Plain. The effluent o:["this plant is discharged into the Indian Ocean. In residential areas on the Darling Plateau, inputs of nitrogen from septic tanks can account for up to 80% of total areal inputs (Gerritse et al., 1992). Soils of the Darting Plateau are predominately lateritic, resulting from weathering of granites and gneisses. A typical profile consists of an upper zone of ironstone gravels cemented together. This zone, termed "caprock", can be up to 2 m thick, and is often overlain by loose sandy gravels ranging in depth from a few centime:tres to more than a metre. The upper zone is followed by a "mottled" zone, consisting of brown to red mottled kaolinitic clays, sometimes containing fe~ruginous gravels in the upper part. This zone gradually changes into a "pallid" zone of generally white kaolinitic clays. The pallid zone grades down into less weathered rock and eventually to fresh rock at depths of up to 60 m. Low lying areas (valley floors) are prone to water logging during extended rain periods, which occur in the winter, and are only marginally suitable for siting septic tanks. Particularly during the winter, rapid *Author to whom all correspondence should be addressed.

leaching to streams of nitrogen and phosphorus may occur in these areas. This paper quantifies nitrogen losses from leaehate of a domestic septic tank into soils of the Darling Plateau. Bromide was added to a septic tank as a conservative tracer. A n d losses of nitrogen were estimated from the decrease in the ratio of nitrogen and bromide in groundwater, downgradient of the septic tank. A comparison is made with nitrogen losses, reported in the literature and calculated from changes in 15N/14Nisotope ratios of nitrate in groundwater affected by leachates from septic tanks. MATERIALS AND METHODS Study site description Movement of nitrogen from a domestic septic tank system in groundwater was studied in an area near a stream (Mahogany Creek) in the Shire of Mundaring (31°58'S,116° 12'E). The area of the catchment affecting the stream is about 82 ha and landuse in the catchment is mainly residential (40% of catchment area with 85 houses). The remainder consists of native bush land (33%), pasture (20%) and disused orchards (7%). The actual study site is a residential lot (0.4 ha), adjacent to Mahogany Creek. The soil environment consists of gently undulating terrain with weU-drained, shallow to moderately deep gravelly brownish sands, pale brown sands and earthy sands overlying lateritic duricrust (King and Wells 1990). Waste water from bathrooms, kitchen, laundry and toilets of a house on the lot is discharged into a septic tank system, consisting of two compartments with a total volume of about 4 m3 connected to a leach drain. The septic tank had been in use since 1987. Groundwater downgradient of the leach drain of the septic tank was monitored from July to November 1993. Until and during that time two adults and three children occupied the house. Effluent from the septic tank leaches into the soil from a

2055

2056

Robert G. Gerritse et al. A, B, C, 0, Eand F bore row

2O

II = - ~ , , w born (t*-50 m ) i " d i n bo,, ( " ~ 0 cm)

Br

,Lg

~

D

s,~c n . k - / ~ Cap rock surface -D- 1 m

E

. . . .

creek F

I

l Well

" 10m

! 10 0=

0

P30rn

10 20 effluent ( m3)

D 50m

30

P 70m NOT TO SCALE

Fig. 1. Schematic view of a site in the Shire of Mundaring, where the fate of nitrogen in the pollution plume from a domestic tank system was studied. drain constructed of open ended concrete blocks. Concrete slabs cover the leach drain, which is embedded in a layer of sand on top of the lateritic soil profile. The layer of sand extends to about 8 m downgradient and is surrounded by a clay bund. The leach drain is 12 m long, 0.6 m wide, 0.6 m deep and at right angles to the surface gradient. Surface fall from the leach drain to Mahogany Creek is about 5 m, resulting in a gradient of about 5%. The leach drain is at a distance of about 80 m from Mahogany Creek (Figs 1 and 2). From July to September the shallow layer of caprock is saturated and a water table forms above the caprock after rainfall events. A network of 26 fully slotted monitoring bores (45 mm PVC tubing) was installed to depths of 0.14-0.9m at distance of 1, 10, 30, 50 and 70 m downgradient from the leach drain. A schematic view of the study site and a plan of the groundwater monitoring bores in relation to the septic tank system are shown in Figs 1 and 2. Bromide tracer experiment

Twenty kilograms of potassium bromide were dissolved in 501 of tap water and flushed into the septic tank system. Chloride, bromide, nitrate, nitrite and ammonium were

Fig. 3. Cumulative amounts of bromide and effluent, leached from a septic tank after addition of 20 kg of potassium bromide. Cumulative bromide is expressed as a percentage of the total amount added at effluent = 0 m 3. monitored weekly from July 1993 to October 1993, in sets of bores placed downgradient of the leach drain (Figs 1 and 2). Bromide behaves conservatively in soils and is not adsorbed (Gerritse and Adeney 1992). And when bromide is added to a septic tank, the ratio of inorganic nitrogen to bromide in the leachate will be transmitted unchanged through groundwater to bores downgradient of the leach drain as long as there are no sinks or sources for nitrogen. Loss of nitrogen was estimated from the decrease in the groundwater ratio of inorganic nitrogen and excess bromide from the septic tank. Excess bromide was calculated from the difference between the measured concentration of total bromide and the calculated background concentration. Background concentrations are calculated from the mass ratio of chloride to bromide in groundwater (-~ 380, Fig. 4) and observed chloride concentrations. Chemical analyses

Nitrate, nitrite, bromide and chloride were determined by ion-chromatography (Gerritse and Adeney 1985). Ammonium was analyzed colorimetricaily with an Autoanalyzer (Technicon) using standard methods. RESULTS AND DISCUSSION

L e a c h drain ~_.Mahogany

•

•

• F Row~

Potassium b r o m i d e was flushed into a septic t a n k on day 219 (Fig. 4). Subsequent b r e a k t h r o u g h o f b r o m i d e in the leach drain is s h o w n in Fig. 3. Metered household c o n s u m p t i o n o f water a n d rainfall over the

Pond

• •o•

•

•

E Row

Well •

oe•e

O

DRew

Leach drain

Septic 01020 ! I Metres

B Row

~us~B! • •

....

_ ~ _ ___~tm_a m

II ~\~---- 70 m II/~_~30%

0 ~

i

•

o 350 300 250 200 150 1~ 100 50!

•

ARow

Fig. 2. Location of groundwater monitoring bores relative to a domestic septic tank system and neighbouring creek at an experimental site (Fig. 1) in the Shire of Mundaring. • = monitoring bores, • = stream water sampling sites.

100 140 180 220 260 300 340 Days of the Year

Fig. 4. Breakthrough of bromide, expressed as the average change in chloride to bromide ratio in rows of bores downgradient of a domestic tank system (Fig. 2). Bromide was added as 20 kg of potassium bromide, dissolved in 501 of water, to the septic tank on Day 219. Distances of each row of monitoring bores from the leach drain are indicated in metres.

Nitrogen losses from a septic tank

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Table 1. Averages of pH, concentrations of nitrogen species and chloride and of chloride to bromide mass ratios in groundwater and surface water surrounding the leach drain of a domestic septic tank installation. The CI/Br ratio was averaged for the period after breakthrough of added bromide in the bores (Fig. 4). Bores are numbered from east to west

(Fig. 2) B~res

A1 Leach Drain B1 B2a B2b B3 CI C2a C2b C3a C3b C3¢ C3d C4a C4b C4c C6 13,1

N-NO

2

N-NO 3 N-NH 4 Averages (mg/l)

N-Total

pH --

Cl

1 140 70 100 100 20 2 1.5 0.5 1 I 1.5 2 2.5 1.5 2.5 0.5 0.3

6.4 7.7 7 6.1 6.5 6.2 6.5 6.7 6.8 6.9 7 6.8 6.9 6.9 7 6.7 7.1 6.7

15 290 250 240 230 80 100 155 150 150 140 140 150 200 200 190 90 110

Cl/Br --

0 40 5 5 10 5 0.02 0.3 0 0.1 0.2 0.2 0.5 0.1 0.001 0.05 0.01 0.1

0.3 0.2 30 70 60 10 2 0.3 0.2 0.3 0.2 0.5 0.3 1.5 1 1.5 0.2 0.05

0.1 100 30 25 35 5 0.03 0.2 0.03 0.02 0.2 0.2 0.5 0.2 0.05 0.2 0.01 0.05

D,2

0.01

0.5

0.5

I

7.6

I I0

60

13,3 13,5 E2 E3 E4 E5 F1 F2 F3 Mahogany Creek downstream Mahogany Creek upstream

0.05 0.01 0.02 0.01 0.01 0.02 0.01 0.05 0.01 0.001

0.1 0.2 0.01 0.005 0.02 0.1 0.02 0.15 0.02 0.3

0.01 0.002 0.01 0.005 0.01 0.01 0.005 0.01 0.02 0.002

0.4 0.5 0.5 0.5 0.6 0.4 0.3 0.5 0.6 0.6

6.7 7.1 6.2 6.4 6.6 6.6 6.8 6.6 6.7 7.1

150 90 100 80 60 135 150 125 70 150

50 130 150 100 150 120 280 160 220 290

0.02

0.3

0.003

0.7

7. I

150

290

first 50 days o f t]ae experiment were 32 m 3 a n d 215 m m respectively. Use of water outside the house was negligible a n d effluent from the leach drain was t a k e n to be equal to the metered volume of c o n s u m e d water. A first incre~Lse in b r o m i d e in the leach drain is observed (Fig. :3) after 3 - 4 m 3 of effluent has leached into the s o i l approximately the volume o f the septic t a n k system. B a c k g r o u n d c o n c e n t r a t i o n s o f b r o m i d e in the leach d r a i n are low (0.5-0.6 mg/l) a n d need n o t be corrected for.

Groundwater C o n c e n t r a t i o n s in groundwater, d o w n g r a d i e n t o f the leach d r a i n (Figs 1 a n d 2), o f nitrogen species, chloride a n d protorts in each set o f bores fluctuated b u t showed n o significant trend during the course o f the experiment. Changes in the c o n c e n t r a t i o n o f b r o m i d e were easily m o n i t o r e d a n d b r e a k t h r o u g h o f b r o m i d e in the bores is s h o w n in Fig. 4. Averages o f pH, c o n c e n t r a t i o n s o f nitrogen species a n d chloride a n d o f the chloride to b r o m i d e ratio in the leach drain a n d in g r o u n d w a t e r m o n i t o r i n g bores are s h o w n in Table 1. Averages o f chloride to b r o m i d e ratios are only for the period after breakt h r o u g h o f added b r o m i d e (Fig. 4). Septic tank effluent is strongly diluted with groundwater. Breakt h r o u g h o f bromide:, a d d e d to the septic tank, indicates a g r o u n d w a t e r velocity above the caprock as

-3.5 6 3.5 4 -80 45 50 40 30 30 30 35 50 35 60 130

high as 3.5 m / d a y during July a n d A u g u s t (Fig. 4). T e m p e r a t u r e in the soil above the caprock was a b o u t 18°C.

Loss of nitrogen N i t r o g e n leaches from the septic tank into the soil mainly as a m m o n i u m a n d is then oxidised to nitrate ( A r a v e n a et al., 1993). Loss o f nitrogen from the leachate o f the septic tank in the shallow aquifer is m o s t likely due to denitrification, as volatilisation of a m m o n i a would only account for a small part o f the decrease in ratio o f inorganic N to b r o m i d e in g r o u n d w a t e r (Table 2). The fluctuating rater table above the caprock a n d relatively aigh t e m p e r a t u r e (18 °C) would also induce a fluctuating partial anaerobiosis in the soils in the vicinity o f the leach drain (Leffelaar, 1987), creating ideal conditions for nitrogen removal ( M c C a r t h y a n d H a u g 1971). Table 2 shows t h a t the ratio o f inorganic nitrogen to excess b r o m i d e decreases from a b o u t 1 in the leach drain to a b o u t 0.2 in g r o u n d w a t e r within 10 m d o w n gradient. It is assumed t h a t a m m o n i u m is n o t significantly retained by the soil a n d t h a t nitrate is n o t a d s o r b e d at all. Even if a m m o n i u m was retained in the soil this would n o t greatly affect the interpretation o f the d a t a in Table 2, as a m m o n i u m contributes only 2 0 - 3 0 % to total inorganic N in groundwater. As

Robert G. Gerritse et al.

2058

Table 2. Comparison of average ratios of inorganic nitrogen to added bromide in the leach drain of a septic tank system with ratios in groundwater monitoring bores after breakthrough of bromide. Data in brackets are standard deviations

Distance from leach drain

(m) 0 I

10 30

Number of samples

10 24 120 40

Mass ratio of inorganicN to excess bromide 1 (0.35) 1.5 (0.6) 0.2(0.3) 0.3 (0.5)

there were no other sources of nitrogen (e.g. fertiliser input), it appears that at least about 80% of nitrogen is lost within the first 10 m from the leach drain. Data in the literature (Aravena et al., 1993, K o m o r and Anderson 1993) on ISN/laN isotopic ratios of nitrate in groundwater polluted by domestic septic tanks in areas at much higher latitudes indicate significant enrichment in the heavier isotope. The data seem to indicate that nitrogen losses are much less than were observed in the bromide tracer experiment, shown in Table 2. A quantitative interpretation of the data from Aravena et al., (1993) and K o m o r and Anderson (1993) in terms of denitrification and/or volatilisation is not possible, however, because background concentrations of nitrate from other sources in groundwater were high and mixing of the septic tank plume with groundwater was not calculated.

Acknowledgements--This study was supported by the Water Authority of Western Australia and the Shire of Mundaring.

Total bromide 120(65) 64 (35) 6(4) 1.7 (1)

Nitrate N (rag/I) 0.2(0.3) 60 (30) 0.8(1.5) 0.1 (0.2)

AmmoniumN 100(15) 30 (20) 0.2(0.3) 0.03 (0.07)

REFERENCES

Aravena R., Evans M. L. and Cherry J. A. (1993) Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems. Ground Wat. 31, 180-186. Gerritse R. G. and Adeney J. A. (1985) Rapid d e t e r m i n a t i o n in water of chloride, sulphate, sulphite, selenite, selenate and arsenate among other inorganic and organic solutes by ion chromatography with UV detection below 195 nm. J. Chromatogr 347, 419-428. Gerritse R. G. and Adeney J. A. (1992) Tracers in recharge-effects of partitioning in soils. J. Hydrol. 131, 255-268. King P. D. and Wells M. R. (1990) Darling Range rural land capability study. Land Resources Series 3, Western Australia Department of Agriculture. Komor S. C. and Anderson H. W. (1993) Nitrogen isotopes as indicators of nitrate sources in Minnesota sand-plain aquifers. Ground War. 31,260-270. Leffelaar P. A. (1987) Dynamics of partial anaerobiosis, denitrification and water in soil: experiments and simulation. Ph.D. Thesis, Wageningen Agricultural University, Wageningen, The Netherlands. McCarthy P. L. and Haug R. T. (1971) Nitrogen removal from wastewater by biological nitrification and denitrification. In (Edited by Sykes G. and Skinner F. A.) Microbial Aspects of Pollution. Society of Applied Microbiology, Symposium Series, 1, pp. 215-232, Academic Press, New York.