Behaviour of nitrilotriacetic acid during groundwater recharge

Water Research Vol. 15. pp. 121 to 128 © Pergamon Press Ltd 1981. Primed in Great Britain

0043-1354/81/0101-0121102.00/0

BEHAVIOUR OF NITRILOTRIACETIC ACID DURING GROUNDWATER RECHARGE J. HRUnECand W. VAN D~FT National Institute for Water Supply, P.O. Box 150, 2260 AD Lcidschendam, The Netherlands (Received June 1980)

Abstraet--A pilot plant study was performed to investigate the behaviour of nitrilotriacetic acid (NTA) during artificial groundwater recharge of river water. The study was designed to investigate the removal of NTA under conditions of artificial groundwater recharge in The Netherlands. The conditions are characteriz~ by relatively low recharge rates and mostly by anaerobic groundwater environments in the aquifers. A furtl~r objective of the study was to obtain information on the possible mobilisation of heavy metals from the soil of the aquifer as a result of the formation of NTA-metal complexes. The results suggest that complete removal of NTA during percolation can be expected for concentrations up to 2 mg NTA 1- t in the surface water, even during the period of low water temperatures. The mobilisation of trace elements from the aquifer during the percolation of water containing these lob concentrations of NTA was not detected and is considered improbable.

INTRODUCTION

The projected substitution of trisodium nitrilotriacetate (NTA) for phosphate in detergents will lead to the presence of NTA in fresh water sources. Intensive research has resolved many conc~,ns about environmental and health hazards asr,ociated with large scale use of NTA as a component of de~crgents. Nonetheless there are some aspects of NTA about which little information is available at present. Inadequate data exist on the consequences of the presence of NTA in surface water and groundwater sources for drinking water supply with regard to the removal of NTA, or of NTA-metal chelates by different water treatment processes, on the influence of NTA on the treatment processes themselves and on health effects of residual concentrations of NTA or NTA-metal complexes in drinking water after a long period of ingestion. About 40~ of the volume of surface water used for drinking water supply in The Netherlands is infiltrated in the dune area along the North Sea coast as a part of its treatment. There has been proposed also a project for artificial recharge of water of the River Rhine in the central part of the country, the Veluwe area, with a capacity of a few hundred million m 3 per yr. Therefore, the fate of NTA during and after artificial recharge is of special interest. The questions needing answers were: what degree of removal of NTA can be achieved by sorption in the ground and by biodegradation during percolation and to what extent can NTA cause mobilisation of metals from the soil due to the formation of soluble chelates. A number of previous studies have shown that NTA is biodegraded fairly rapidly under aerobic con121

ditions and at higher water temperatures in sewage treatment systems and in surface waters (Shannon et al., .1974; Prakhash, 1976; Matheson, 1977). Similarly, degradation of NTA during percolation of water through soil under aerobic conditions has been detected (Klein, 1974; Tiedje & Mason, 1974). Conflicting findings have been reported about the effect of temperature and/or redox potential on the rate of biodegradation of NTA. Results of a study on the temperature effects during treatment of waste water in activated sludge units carried out by Eden et al. in 1972, were that only 3°.o removal of the initial concentration of NTA occurred at a temperature of 5°C while 66-82% was achieved at 7.5°C. Gudernatsch (1974) concluded on the basis of his experiments that no degradation could be expected at temperatures below 5°C in the River Rhine. On the other hand data from the Canadian monitoring program indicate that even during severe Canadian winters NTA is effectively degraded in sewage treatment plants and in surface water (Prakash. 1976; Matheson, 1977). Related to the effect of temperature on NTA degradation, the results of the study of Tiedje & Mason (1974) show that an efficient removal of NTA at low temperature (2°C) was achieved during soil percolation after an acclimatisation of soil micro-organisms at a higher temperature (12°C). One of the most significant factors effecting NTA biodegradation seems to be the redox potential. A number of studies (Klein, 1974; Dunlap et al.. 1972: Tiedje & Mason, 1974) indicate that when oxygen is absent, NTA degradation durihg percolation of water through the ground is very slow. However, according to Enfors & Molin (1971), degradation also occurs

122

J. HRu.Ec and W. VAN DELft

under anaerobic conditions, if nitrate is present in the water as electron acceptor. Another concern about the environmental impact of NTA is related to the possible mobilisation of heavy metals from the bottom sediments of inland waters and from the soil during percolation of water containing NTA as a result of the formation of N T A metal complexes. Several authors have found that NTA in ppm concentrations can release heavy metals from sediments (Chau & Shiomi, 1972; Zitko & Carson, 1972; Banat et al., 1974). According to other authors (Prakash, 1976; Singer, 1977) the effect of expected residual concentrations of NTA (max. concentration of the order of mg l - i) on mobilisation of heavy metals from sediments will be negligible. The purpose of the present study was to determine the fate of NTA during artificial groundwater recharge. In particular the following points have been considered: (a) removal of NTA during percolation through the ground during periods of low water temperature and under anaerobic conditions, i.e. under the circumstances of groundwater recharge in The Netherlands; (b) mobilisation of heavy metals from the soil during percolation of water with low concentrations of NTA. MATERIALS AND METHODS Facility

The study was carried out in a pilot plant at Leiduin, a dune area along the west coast of Holland, where water of the River Rhine is infiltrated by the Amsterdam Water Works (Zoatcman et al., 1976). A schematic diagram of the pilot plant is shown in Fig. I. The plant consisted of a stock-tank for Na-NTA solution (101.) a constant-headtank (401.) and two recharge filters` each 6.5 m high and 2 m in diameter. The first filter was filled with dune sand and the second one with sand from the Veluwe area in the central part of The Netherlands. The sands were analysed for particle-size distribution, some chemical parameters, cation-exchange capacity and metal contents. From the sieve analysis the effective diameters (dto) of the dune and Veluwe sand was determined as 0.11 and 0.17 mm respectively. The coefficient of uniformity

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Table 1. Heavy metals detected in sands before recharge Concentration

(#g g- t)

Metal

Veluwe sand

Dune sand

Cadmium Cobalt Chromium Copper

0.05 1.I 1.1 0,7

0.04 1.3 3.0 1.4

Lead Nickel Zinc

1,5 2.4 3.5

2.0 4.3 lI

(d6o/dlo) of the dune sand was 2.06 and of the Veluwe sand was 2.35, The cation-exchange capacity of the Veluwe sand as estimated with BaCI2-TEA (pH 8.2) was 0.78 meq 100g- t and that of the dune sand was 1.98 meq I00 g-L. Data on the heavy metals detected in the sands are presented in Table 1. The permeability of the dune sand bed was 9 m day- t and of the Veluwe sand bed was 22 m day - ~, at bed porosities of 37-39%. The water used during the experiments was Rhine water which is being coagulated and filtered at the place of abstraction in Jutphaas and is then being transported to the dune recharge area of the Amsterdam Water Works. NTA was injected from a stock solution of Na-NTA into the influent of the constant-level-tank (Fig. 1) to obtain a final concentration of 1 mg NTA 1- ~. This concentration is more than twice as much as the predicted N T A concentrations in the water of the Rhine during large scale application of N T A in the catchment area of the river (Golterman & van Weelden 1976). The recharge rate,0.25 m s m -2 per day, which was used during the tests,is similar to that of dune recharge practice in The Netherlands,

The detention time of water in the sand beds was about 6 days. To limit the detention time of water above the surface of the sand bed as far as pos~ble and thus to restrict biodegradation of N T A prior to percolation, the water depth above the surface of the bed was kept at a minimum (c. I0 cml. To restrict biological activity in the water above the surface of the sand bed, the tanks were covered with aluminum plates. The experiments were started in November 1976 and finished in M a y 1978. The investigation could be divided in three periods: (a) November 1976--February 1977 {aerobic recharge at low water temperature).

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Behaviour of nitrilotriacetic acid during groundwater recharge 9O

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1977

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Sampling and analysis Samples of the water above the surface of the sand beds and of the tank effluents were analysed weekly for NTA, pH, redox potential, oxygen, alkalinity and turbidity, Twice a month ammonia, nitrate, iron, manganese, COD and trace elements (aluminum, cadmium, copper, chrornium, nickel, lead and zinc) were determined. NTA was measured by the polarographie method developed by Haring & van Delft (1977) which has a detection limit of 50 pg NTA 1- ~. The redox potential was determined with a platinum electrode and calomel reference. The electrode was checked with the redox buffer of Zobell (19461~ The trace elements were determined by atomic adsorption spectrophotometry, employing standard procedures,

RESULTSANDDISCUSSION Period November 1976-March 1977 (Recharge under aerobic conditions at low water temperature) Dosing of NTA began on 3 November 1976.

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Figure 2 shows that NTA was first detected in the effluent after about 2 weeks of recharge and disappeared from the effluent of the Veluwe filter after the 6th week and from that of the dune filter after 8 weeks. It appears that biodegradation became established after about 3 weeks, at a water temperature of 8-10°C, The paths of the breakthrough curves indicate that during the first 3 weeks sorption played a role in NTA removal. On the assumption that the removal by sorption took place only before the maximum of the breakthrough curve was reached and that the detention time in the sand bed was 6 days, the sorption capacity of both sands could be estimated to be about 10-3-10 -" pg NTA g-a sand. A negligible sorption of NTA on the sand had been predicted from the results of the batch shaking tests which have been carried out during the preparation of the pilot plant experiments. During the batch tests 20 g of each of the sands was agitated in 250 ml water with different concentrations of NTA (0.25--50 mg 1- ;) during a period of 3 days at 5°C. No detectable sorption was found by this method. After the acclimatisation time of 6--8 weeks no NTA was detected in the effluents of the filters, even during the period from mid-December 1976 to the beginning of February 1977 with a water temperature of 3-5°C. In order to obtain information on the fate of the NTA degradation in the deeper strata of the sand bed, samples were abstracted at different depths in the bed. The results presented in Fig. 3 indicate that NTA was not detectable in the Veluwe filter at a depth of l0 em below the bed surface. In the dune filter the NTA penetrated deeper. Judging from the observed concentrations of dissolved oxygen, biological activity was generally lower in the latter filter. The reason might be the fact that the Veluwe filter was filled with sand nearly 1 year before the start of the experiments and a biological activity had developed there due to thepercolation of rainwater. The experiment on the other hand with the dune sand was started with fresh sand.

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Behaviour of nitrilotriacetic acid during groundwater recharge

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Period March-November 1977 (Recharge under anaerobic conditions at higher water temperature) Due to the relatively low concentrations of organic matter and ammonia in the recharge water it was necessary to dose a reductant in order to achieve anaerobic conditions in the sand beds. In mid-March 1977 the dosing of 7 mg glucose 1- J began. This low dose was used in order to prevent too low redox potentials, i.e. to create the conditions which prevail in dune recharge practice. After 3 weeks

from the start of the dosing, complete disappearance of oxygen was achieved (Fig. 4) and the redox potential dropped at the same time to less than 300 mV (Fig. 5a). Starting from mid-April, slight dcnitrification was detected (Fig. 5b, c). The behaviour of manganese was not too clear but a slight increase of Mn in the effluents can be seen in Fig. 5(d) and (e). No reduction of iron was observed during the whole period of the experiments. The dosing of glucose prorooted the growth of the bacterium Sphaerotilus

J. HRUnECand W. VAN D~Fr

126

Table 2. Trace elements in the influent and the effluents of the recharge filters Element (/~gl-~) AI Cr Cu Ni Pb Zn Cd* Cd'l" Cd:[:

N

lnfluent X

18 18 19 18 19 10 19 8 10

24.2 3.1 4.7 3.4 1.9 76 0.20 0.1 0.30

SD

N

14.6 1.62 4.33 1.34 1.10 88.9 0.11 0 0.09

17 "18 19 19 19 18 19 8 10

Efflumat Dune filter X SD 18,9 1.0 2.4 2.2 1.8 16 0.35 0.225 0.26

N

Effluent Veluwe filter .~ SD

11.4 :. 18 0.53 18 1.12 19 0.85 19 1.17 19 7.0 19 0.06 19 0.046 8 0.10 8

18.0 0.65 2.7 2.5 1.8 13 ' 0.30 0.175 0.25

14.0 0.41 1.25 1.5 1.15 8.2 0.07 0.046 0.09

* 28 February-25 October 1977. f 25 October-13 December 1977. ~:10 January-8 May 1978. N--number of samples; ,f--arithmetic mean; SD--standard deviation.

natans in the constant-waterlevel-tank; this created problems with clogging of the pipes and valves, Consequently, 12rag 1.-1 of sodium acetate was dosed from May 1977. To guarantee the presence of a source of nitrogen in the recharge water in addition to NTA 1.5 mg NH,CI 1-1 was dosed. Despite the anaerobic conditions in the sand beds the NTA concentrations in the effluent remained below the detection level. Changes in conc*ntrations of the trace elements during the recharge are presented in Table L Table 2 shows that there was a substantial removal of Zn and some removal of AL Cr, Cu and Ni, no removal of Pb and a slight increase in Cd. These data, except for those relating to Cd, are in good agr~ment with the results of previous experiments on recharge of river water without NTA, which were carried out in the Leiduin pilot plant from 1972 to 1976 under similar conditions (Hrubec, 1979). Because of the slight increase in Cd concentration in both effluents after recharge and because no changes of Cd concentrations during the percolation through sand were observed during the tests with the recharge of water without NTA, mobilisation of Cd by NTA was assumed as a potential risk factor. To prove this, dosing of NTA was stopped for two months (25 October-18 December 1977), the increase in Cd concentrations after recharge continued (see Table 2). We did not succeed in explaining this phenomenon. Period December 1977-May 1978 (Recharge under anaerobic conditions at low water temperature) Dosing of NTA started again on 18 ~ b e r 1977. Figure 6 indicates that within 1 month NTA was not detectable in the effluents. The differences in the concentrations of NTA in the effluents were probably caused by partial clogging of the influent pipe of the Veluwe filter during Christmas days so that the recharge rate decreased to about 5 cm d a y - 1 Relatively rapid acclimatisation was achieved despite the low water temperature (4-7°C). From mid-February 1978 the NTA concentration of the influent water was

increased to 2 mg 1-1. This increase had no influence on the removal and the NTA concentrations of the effluents remained less than the detection limit. The observed easy degradation of NTA during anaerobic conditions in the sand beds can be explained by the facts that the upper layers of the sand beds were aerobic and that the degradation of NTA during passag¢ through the beds was completed before the anaerobic Myers were reached. In Fig. 7 the pattern of dissolved oxygen within the sand beds is presented. Comparison of the oxygen patterns in winter and summer shows that the aerobic layers during the period with low water temperature are deeper than during the period with higher ternperatureg It seems possible that the lower rate of biodegradation at low temperature is compensated due to proiongation of the d¢tention time of NTA in the aerobic layer during the winter time. In dune recharge practice the thickness of the aerobic layer in the sand is usually greater than in the pilot filters and therefore, the conditiong for NTA degradation are more favourable. Increasing the NTA concentration in the influent to 2 mg l-1 had no influence on the fate of the trace elements, which was the same as during the previous

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Behaviour of nitrilotriacetic acid during groundwater recharge

127

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TEMP. WATER 19°C

Fig. 7. Oxygen concentrations in different depth of sand bed {winter × summerl.

periods. Only a decrease of Cd concentration after recharge was detected (see Table 2). Even if the behaviour of Cd during previous periods, when Cd concentrations in the effluents of the filters were higher than in the influent with and without dosing of NTA, is taken into consideration, it seems unlikely that the Cd increase was caused by NTA.

4. CONCLUSIONS

This study supports the following conclusions: (1) Complete removal of N T A at concentrations up to 2 mg NTA 1-~ can be expected during artificial recharge of surface water into sandy soil under aerobic and slightly anaerobic conditions in the aquifer, i.e. in a situation which prevails in the groundwater recharge practice in The Netherlands. This complete degradation can also be achieved at low water temperature (3-5°C). (2) The acclimatisation of the biological processes in the soil which are responsible for NTA removal can take a few weeks. Sorption processes on the sandy soil does not play an important role in NTA removal. Only a restricted retardation of the breakthrough of NTA can be expected during the first period of the recharge due to sorption processes. (3) The mobilisation of trace elements from the recharge aquifer as a result of percolation of water containing NTA up to a concentration 2 mg NTA l -~ is improbable. Acknowledgements--The authors wish to thank the staff members of the laboratories of the Chemical Biological Division of the National Institute for Water Supply for analyses of water and sand, and Mr R. Westerveld of Amsterdam Water Works for technical assistance. They would also like to express their appreciation for the support of this study by the management staff of the Amsterdam Water Works.

REFERENCES

Banat K., F~Srstner V. & Miiller G. (19741 Experimental mobilisation of metals from aquatic sediments by nitrilotriacetic acid. Chem. Geol. 14, 199-207. Chau Y. K. & Shiomi M. T. (19721 Complexing properties of nitrilotriacetic acid in the lake environment. War. Air Soil Pollut. 1, 149-164. Dunlap W. J., Cosby R. L., McNabb J. F.. Beldsoe B. E. & Scalf M. R. (1972) Probable impact of NTA on ground water. Ground War. 10, 107-117. Eden G. E., Culley G. E. & Rootham R, C. (1972) Effect of temperature on the removal of NTA during sewage treatment. Water Res. 6, 877-883. Enfors S. O. & Molin N. (19791 Anaerobic degradation of nitrilotriacetate (NTA) by bacteria. Vatten 27, 162-163. Golterman H. L. & van Weelden R. H, (1976) Schatting NTA-concentratie in Nederlandse oppervlaktewater bij vervan~in~z van fosfaten uit wasmiddelen. H20 9. 57-58. Gudernatsch H. (1974) Biologischer Abbau yon Nitrilotriessig SaiJre. Gewiiss. Abwiiss. i15, 418--421. Haring B. J. A. & van Delft W. (1977) Determination of nitrilotriacetic acid in water b? derivative pulse polarography at a hanging mercury drop electrode. Analytica chim. Acta 94, 201-203. Hrubec J. (1980) Pilot plant study of artificial recharge of pretreated water from river Rhine in the Veluwe area. Bull. nam Comm. Germ. Int. Commn Irrig. Drainaqe. In press. Klein S. A. (19741 NTA removal in septic tank and oxidation pond system, d. Wat. Pollut. Control Fed. 46. 78-88. Matheson D. H, V. (1977) Nitriloacetic acid (NTA) in the Canadian environment. Scientific Series No. 74. Inland Water Directorate, Ottawa, Canada. Prakash A. (19761 NTA (nitrilotriacetic acid~---an ecological appraisal. Report No. EPS-3-WP-76-8. Environmental Protection Service, Environmental Canada, Shannon E. E.. Fowlie P. J. A. & Rish R. J. (1974) A study of nitrilotriacetic acid (NTAt. Degradation in a receiving stream. Report No. EPS-4-WP-74-7, Environmental Protection Service. Environmental Canada, Singer P C. (1977) Influence of dissolved organics on the distribution, transport and fate of heavy metals in aquatic systems. In Fate of Pollutants in the Air and Water Environments (Edited by Suffer E.), VoL 8. Part 1, p. 155. Advances in Environmental Science and Technology, Wiley, New York.

128

J. HauaEc and W. VAN DELFT

Tiedjc J. M. & Mason B. B. (1974) Biodegradation of nitriIotriacetate (NTA) in soils. Soil Sci. Soc. Am. Proc. 38, 278-283. Zitko V. & Carson W. V. (1972) Release of heavy metals from sediments by nitrilotriacetic acid. Chemosphere 3, 113.

Zob¢ll C. E. (1946) Studies on redoxpotential of marine sediments. Bull. Am. Ass. Petrol. Geol. 30, 477-513. Zoeteman B. C. J., Hrubec J. & Brinkmann F. J. J. (1976) The Veluwe artificial recharge plan. Water quality aspects. J. Inst. Wat. En.qrs. Scient. 30, 123-137.