COMMUN. SOIL SCI. PLANT ANAL., 33(5&6), 695–709 (2002)
FEASIBILITY STUDY OF LONG-TERM CONTINUOUS FIELD MEASUREMENT OF SOIL REDOX POTENTIAL G. Eshel* and A. Banin** Department of Soil and Water Sciences, Hebrew University, P.O. Box 12, Rehovot 76100, Israel
ABSTRACT An in situ and continuous redox potential measurement protocol for soils and groundwater was optimized, as part of a research that assays the redox potential changes in recharge basin soils during wastewater reclamation using the Soil Aquifer Treatment (SAT). The results show that it is possible to measure continuously and reliably the redox potential in the soil profile of the recharge basins and in the groundwater by using a combined PtkAgjAgCl cell (electrode), a specialized interface, and a portable data-logger. Redox potential values measured before true local equilibration of the electrodes was reached are interpreted as “mixed-potentials” strongly biased by the presence of oxygen near the electrode. Uninterrupted and non-disturbing measurements were carried out in the field for relatively long periods (few days to few weeks) giving reliable information on the temporal cycling of the redox *Current Address: Department of Land, Air and Water Resources, University of California, Davis, CA 95616. **Corresponding author. E-mail:
[email protected] 695 Copyright q 2002 by Marcel Dekker, Inc.
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potential in the soil profile during wastewater recharge cycles. The method was found to be dependable, non-expensive and easy to apply.
INTRODUCTION The redox potential (Eh or pe) is an important parameter in many biogeochemical processes in soils, sediments, surface water bodies and groundwater. Oxidation –reduction potential (ORP) measurements in general and in particular in soils, are considered to be complex and not always reliable (1 – 4). The classical approach is to measure directly the potential difference in an electrochemical cell employing a sensing platinum electrode, a reference electrode, such as a calomel electrode and a high input-impedance potentiometer (1,4,5). One of the main problems revealed during the last three decades was the large variability of directly measured ORP values in soils and groundwater. Variability was observed both using several electrodes in proximity to one another and in repeated measurements using the same electrode. Stumm and Morgan (3) attributed this variability to the fact that the measured environment is heterogeneous (microniches) and to the lack of equilibrium among the chemical redox couples (resulting in what is called mixed potentials). In a well publicized paper, Lindberg and Runnells (6) examined numerous results of field measurements of ORP in freshly pumped groundwater using the Pt electrode, and compared them to accurate chemical analyses of the concentration ratios of redox couples in the water. Their results showed almost no correlation between the redox potential measured directly in the water and that computed from concentrations of the individual redox couples. Whereas concentration-based computations predicted values between 2300 and þ1000 mV [pe ø (25) – (þ16)], the values measured directly in the field were clustered in a much narrower range of 2150 and þ550 mV [pe ø (22.5) – (þ9.1)]. These results lead the authors to an extreme approach to direct ORP measurements, recommending the discontinuation of the use of the platinum electrode for redox potential measurements in groundwater. Consequently, many researchers turned to the use of indirect and timeconsuming methods by measuring redox couple concentrations in soil solutions and water. Generally, the redox conditions are determined by measuring the ratios of the redox couple components and back-calculation of the expected ORP in situ (7 –10). The measurement is done by solution sampling under non-oxic conditions and analyzing the concentrations of the system components in the laboratory.
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Only sporadic direct measurements of ORP in soils were reported in recent years. Commonly, the approach used is to employ either a homemade or a commercial platinum wire electrode which is inserted into the soil and coupled to a reference electrode to form an electrochemical cell. Wide variation is reported with regard to the time allowed for electrode “equilibration”. It varies from 5–10 minutes according Ponnamperuma (1), 10–15 hours according to Norrstom (11) or overnight, according to Patrick et al. (5). Still, Cogger et al. (12) could relate the variability of direct ORP measurements in soils to the microscopic spatial variability. They claim that ORP measured by using the platinum electrode may not be accurate but still gives a general idea on the redox state of the soil and thus can be used qualitatively for identifying iron speciation in flooded soils. The indirect methods, although supplying detailed information on the solution chemistry and the redox conditions in the system, are tedious, lengthy, expensive and time-consuming. Furthermore, they disturb the site from which the sample is being taken. They are particularly limited when it is necessary to follow rapid temporal changes in a given soil horizon. On the other hand, if the direct electrode measurement is improved, it will enable rapid non-disturbing measurement of the redox potential on a continued basis in the field. A comprehensive research project examining the role of the soil as an adsorbing filter for trace heavy metals in the Soil Aquifer Treatment (SAT) system for wastewater reclamation, brought up the question of redox potential changes occurring in the soil over time (13). The SAT system is employed on a large scale in the Shafdan Plant near Tel-Aviv, Israel (14,15) and other sites (16). Raw municipal wastewater is treated in the Shafdan plant either in facultative oxidation ponds or in a mechanical – biological plant using an activated sludge process involving nitrification/denitrification steps. As a final step in the process, the recylced effluents are recharged into the local aquifer, using sandy soil recharge-basins for polishing and storage in what is called the “Soil-Aquifer Treatment” (SAT). Following aquifer detention, the water is pumped and used for irrigation. Redox potential changes in the soil may cause matrix minerals such as iron oxides to reductively dissolve out of the soil thus lowering its capacity to sorb trace elements. Moreover, reoxidation and secondary precipitation of the dissolved iron deeper in the profile may make the soil impermeable. The changes in the redox potential during recharge, due to the wetting-and-drying cycles and due to the continuous addition of soluble and suspended organic matter, are closely related to the geochemical processes that the soil is undergoing (17,18). There is therefore the need to measure ORP continuously and reliably in the soil profile of the recharge basins during repeated recharge cycles. The objectives of the study presented here were to optimize a protocol for in situ continuous field measurement of redox potential and to assess its advantages and limitations.
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MATERIALS AND METHODS Experimental Site The research took place at the Soreq site of the Dan Region Reclamation Project (SHAFDAN), which is the largest in Israel and treats about 100·106 m3 of municipal sewage water every year. Following secondary treatment, the effluents are recharged into the local aquifer using recharge basins, for a detention period in the SAT system, until they are pumped and reused for irrigation. Recharging operations were initiated at the Soreq site in 1977. Since then, the facilities at this site have been treating effluents at a hydraulic load between 15 to 100 m of effluents per year. A cumulative load of ,1100 m has been recharged in the Soreq basins since the beginning of their operation until 1996 when the present study was initiated (19).
Soils Sandy soils (Dune sand) that were used for effluent recharge during SAT treatment for 19 years in basin 103/4-5 of the Shafdan plant were used for measuring the redox changes. The soil profiles in the basin has mainly sands with low specific surface area, low total organic matter and low calcium carbonate content (Table 1). Measurements were conducted on sampled soils incubated with effluent under controlled conditions in the laboratory (25 ^ 1.58C) or directly in the recharge basin soils on site during recharge effluent cycles.
Redox Potential Measurements Redox potential was measured by specialized solid-state pH/ORP interfaces with high input-impedance (Fourier Systems, Inc., Israel, model DT016) connected to combination ORP electrodes (Cole-Palmer, USA, model P-05990-55) and recorded using a Multilog data-logger (Fourier Systems, Inc., Israel, DB-525, Version 4.3). Each electrode was allocated with its own interface/ probe and logger channel. The ORP electrodes consisted of a platinum measuring electrode and a reference electrode (AgjAgCl with a 3.8 M KCl salt bridge). The measured and recorded potential value (E) is the potential difference between the platinum electrode and the reference electrode. Due to slight intrinsic differences among the electrodes, each interface and its electrode were initially calibrated and adjusted in a pH ¼ 7 buffer solution containing 2 g L21 of quinhydrone (hydroquinone/benzoquinone 1/1 complex) which gives an E value of 87 ^ 2 mV. In addition, each interface and its
0.55
11.8
SSAa ðm2 =gÞ CaCO3 (%) 1.36 ^ 0.70
OMb (%) 0.93 ^ 0.20 98
Sand (%) —
Silt (%) 2
Clay (%)
Texture (Sandy)
Top 15 cm layer, Basin 103/5, Soreq site, Shafdan water treatment plant, Tel-Aviv, Israel. a SSA ¼ Soil Surface Area. b OM ¼ Organic Matter.
Entisol
Soil Type
Hygroscopic Moisture (%)
Table 1. Representative Basin Soil Characteristics
Dull yellow orange 10YR7/4
Soil Color (Munsell) Dry
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electrode were examined in pH ¼ 4 buffer solution containing 2 g L21 of quinhydrone and adjusted to give an E value of 263 ^ 2 mV. This procedure allowed to bring all the ORP electrodes to the same sensitivity scale. Data sampling intervals could be varied. In most cases measurements were taken every minute. Readings were converted from the analog to digital form and were stored in the logger. Data were downloaded periodically from the logger to a portable computer without interfering with the electrodes readings. Due to the difference between the electrochemical cell actually used in the ORP measurement and the standard one used for defining the ORP scale (the standard hydrogen electrode), a correction potential value (ER) has to be added to the actually measured value (E). ER is the measured potential difference between the AgjAgCl reference electrode and the standard hydrogen electrode. The redox potential Eh, in mV, is then obtained by Eh ¼ E þ ER
ð1Þ
ER values depend on the measurement temperature and are supplied by the electrode producer. The pe value, that can be viewed as a measure of the electron activity, ae, in the system (20), is calculated from the Eh as follows Eh ¼
2:303RT pe nF
ð2Þ
where pe is electron activity ðpe ¼ 2log ae Þ; R is the gas constant (1.978 cal deg21 mol21), T is the absolute temperature (8K), F is the faraday constant (23,061 cal volt21 eq21) and n is the number of electrons participating in the reaction. Where n ¼ 1 and T ¼ 258C pe ¼
EhðmVÞ 59:2
ð3Þ
Experiments Under Controlled Conditions in the Laboratory Undisturbed soil cores were sampled by slowly forcing a PVC cylinder (200 mm diameter and 35– 40 cm length) horizontally into the wall of a trench dug to the desired depth in the basin soil. This way a non-disturbed soil sample was taken from a given soil horizon. The PVC cylinder with the undisturbed soil sample was carefully placed vertically in a 10-liter plastic bucket and transported to the laboratory within two hours from sampling. The samples were placed in a constant temperature room (25 ^ 1.58C), ORP electrodes were inserted into the soil at 10 cm depth and the ORP potential recorded as described above. Within two more hours, secondary effluent from station No. 5 at the Shafdan site was
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added to the bucket. For forcing out all the air from the soil and insuring saturation, the effluent was added slowly into the space between the bucket wall and the soil cylinder, such that the water penetrated the soil from the bottom up.
Field Experiment In order to measure ORP in situ in the soil and its solution during flooding cycles the system shown in Fig. 1 was constructed and installed in recharge basin 103/4-5 at the Shafdan plant. ORP electrodes were specially fitted for installation in the field. A 25-mm diameter PVC tubing and an extension coaxial cable were attached to every electrode. These were attached such that the BNC connector and the upper side of the electrode body was inside the PVC tubing and the measuring tip of the
Figure 1. Schematic presentation of the system for simultaneous measurement of redox potential in the soil and its solution and for soil solution sampling.
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electrode protruded out. The electrodes were inserted into a hole drilled in the soil to the required depth using an auger of a size similar to the PVC tubing to ensure good contact between the electrode tip and the soil. Two ORP electrodes were inserted into the soil at a distance of 50 cm from one another and to a depth of 150 cm and secured in place. A third electrode was inserted to the same depth inside the piezometer installed between the two soil electrodes in the same site (Fig. 1). Redox potential values were measured and recorded during effluent recharge and drainage cycles, using the system and procedures described above. Soil solution samples were taken from the piezometer installed in the basin. The piezometer was made from 3 m long 50 mm OD PVC tubing capped at the bottom and perforated over a 15 cm length segment at a height of 10– 25 cm from the bottom. The perforation was covered with a layer of plastic fabric and the piezometer was inserted into a hole drilled to 160 cm depth. A flexible 10-mm diameter tubing inside the piezometer was used to bubble nitrogen gas into the piezometer during the time of the ORP measurements in order to minimize the effect of atmospheric air on the water inside. An additional 10-mm tubing, connected to a 70 mL syringe and a three-way sampling valve, was used for periodic sampling of the soil water from the piezometer. After setting all the above system the top of the piezometer was plugged with a rubber plug to reduce gas exchange with the atmosphere. The first sample volume was transferred into a 50 mL bottle that contained 1 mL of the Ferrozine iron reagent (21). The sample was covered with aluminum foil and stored in a cooling box. Ferrous iron concentration was determined by measuring light absorption at wavelength of 562 nm.
RESULTS AND DISCUSSION Measuring Redox Potential Changes in Soils During Incubation in the Laboratory Replicate curves, showing the change in ORP with time after flooding of non-disturbed soil cores sampled at the Shafdan recharge basin and incubated with effluent under controlled laboratory conditions, are presented in Fig. 2. Different samples show different rates of ORP decrease but all of them start at pe values of ,8 and eventually reach a similar low pe value of 23 to 24. The time required for electrode equilibration varies between ,90 h and ,200 h. It is clear from these results (and many others which have been obtained and are not shown here) that measurements taken after short equilibration times (few minutes to few hours), as customary in many such procedures, do not represent the true redox potential of the soil. Due to the more than proportional kinetic effect of oxygen on
SOIL REDOX POTENTIAL
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Figure 2. Redox potential (pe) changes with time during incubation of a soil sampled from recharge basin 103/4-5 and saturated with effluent under controlled conditions in the laboratory (25 ^ 18C). Different curves show replicate samples. The arrow indicates the time when the ORP electrode was taken out from the soil for one minute and inserted back immediately. A detail of the pe changes following electrode removal and reinsertion is shown in the insert. See text for further discussion.
the Pt electrode (3), the readings are biased towards high pe (oxidized) values. It is suggested that the initial period of poising at pe , 8, observed after insertion of the electrodes and reintroduction of effluents, is due to the presence of O2 from air in the immediate vicinity of the newly inserted electrode. The varying length of this period in the 3 replicates may be related to the different amounts of oxygen (air) occluded during electrode insertion. The short plateau in the pe vs. time curves observed in Fig. 2 at pe ¼ 5 – 6 may be attributed to denitrification and reduction of nitrate in the effluents and/or reduction of soil Mn. To check further the effect of occluded oxygen on the ORP electrode reading in a reduced soil having a pe of 23, the following experiment was done: The equilibrated electrode was carefully pulled out from the soil and reinserted back into the soil after exposure to the atmosphere for just 1 minute. A rapid jump of about 5 units in pe reading was recorded (Fig. 2, curve indicated by arrow and detailed insert). About 30 minutes after reinsertion, it seemed that the measured pe stabilized at a high value of þ1.8 pe units. However, 6 hours later the measured pe started to decrease (insert, Fig. 2). Continuing the measurement for another 100 hours, has shown that the steady state value in the soil is much lower (by about 4 pe units) than the apparently stable value recorded after several hours, and is consistent with the one measured before pulling out and reinserting the electrode.
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These results show that the Pt electrode may measure correctly soil redox potential if permitted to equilibrate with the soil over a rather prolonged period of time. Redox potential values measured in the interim period before true local equilibration of the electrodes was reached are interpreted as mixed-potentials strongly biased by the presence of oxygen near the electrode.
Pt Electrode Response to Mixed Redox Potentials To further demonstrate the phenomenon of mixed-potentials recorded by Pt electrodes, the following experiment was set up. A container was split into two cells and 3 ORP electrodes were installed: one in each cell (R and L) and the third (M) was fixed and sealed in the partition between the cells such that half of its surface faced each of the cells (Fig. 3). First distilled water was added to both cells and the ORP values were recorded. Practically equal values were measured for all three electrodes (Fig. 4a). Then, the R and L cells were filled with 2 g L21 quinhydrone in pH ¼ 4 and pH ¼ 10 buffer solutions, respectively. The R and L electrodes measured the expected redox potentials for the respective standard solutions. The middle electrode (M) recorded, after the initial equilibration period, an intermediate value between those of the two other electrodes (Fig. 4b). A slight bias towards the higher pe in the M electrode may be due to micro-scale differences in the surface exposure of the M electrode to the solution in the R cell containing the higher pe solution. This simple experiment clearly demonstrates the phenomenon of mixed potentials recorded by ORP electrodes.
Figure 3. A schematic presentation of the system for measuring redox potential by an ORP electrode exposed to two cells with different solutions. L, M, R are ORP electrodes.
SOIL REDOX POTENTIAL
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Figure 4. Mixed potentials of ORP electrodes—changes of redox potential with time as measured in the system shown in Fig, 3. (a) Both cells containing distilled water; (b) cells L and R are filled with 2 g L21 hydroquinone in pH ¼ 10 and pH ¼ 4 buffer solutions, respectively.
Feasibility of Reliable Direct Measurements of Redox Potential in the Field The feasibility of continuously recording redox potential changes in the soil profile and in the infiltrating effluent solution in situ during recharge cycles has been tested and demonstrated in the basins. Continuous measurement of ORP was conducted in the field using the system described above (Materials and Methods, Fig. 1). Flooding started at time zero and water level in the basin started to build up (Fig. 5a). The low pe front (reduction front) reached the measuring depth (150 cm) after 60 –70 h of flooding, at both soil electrodes (1 and 2) and after about 100 h in the piezometer (Fig. 5b). Note that the percolating effluent inside the piezometer was maintained in an oxygen-free environment by continuously
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bubbling N2 gas. The same low ORP values [(23) – (24) pe units] were recorded in both the soil and the percolating effluents. Redox potential measured in the piezometer increased steeply as air penetrated the piezometer ,20 h after the end of the drainage of the surface impounded effluent (,200 h, Fig. 5a). It took an additional 50 hours for air to penetrate into the horizon where ORP was measured by the two soil electrodes, and the pe values steeply increased as well (Fig. 5b). The results prove the feasibility of direct, continuous and non-disturbing field measurement of redox potential in reduced soils and in their solutions. The correspondence between the directly measured ORP and the concentration of the affecting chemical redox couple was preliminarily studied in a similar experiment that included sampling of the water from the piezometer (in an air free environment). It was found that the Fe2þ concentration in the solution was 0.003 M and the pe was þ0.3. Both the pe and Fe2þ concentration are within the theoretically expected range for the common iron oxide and oxyhydroxide minerals (pe ¼ 1:9 for Fe-amorp, pe ¼ 2:1 for soil Fe, pe ¼ 21:5 for goethite and hematite) at equilibrium with a solution of a total ionic strength of 0.024 M.
Figure 5. Redox potential measured during a flooding and drainage cycle at basin 103/4-5 in the Soreq site. (a) Changes in the effluent level in the basin; (b) redox potential in the soil profile. Electrodes 1 and 2 installed in the soil and electrode 3 in a piezometer, all at the same depth (150 cm) and adjacent to one another (see Fig. 1).
SOIL REDOX POTENTIAL
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Long-term electrode stability and drift were checked in the field. The ability to calibrate each electrode independently, using separate solid-state interface, enables to bring all the electrodes to the same sensitivity range. Recalibration of electrodes, in the current system, was required only after 10–15 days. The negligible changes (^2 mV) in the ORP electrode calibration even after a number of weeks of insertion to the soil, show the reliability of the electrode and the measuring system used in the proposed protocol. Overall, and in contrast with many other studies (reviewed in the Introduction), the present results prove the feasibility of direct accurate and continuous measurements of redox potential in soil and water, provided the proper precautions are taken to minimize the interference by atmospheric oxygen.
CONCLUSIONS A detailed protocol for in situ measurement of the oxidation – reduction potential (ORP) in soil and water, was tested. It entails continuous measurement by combined Pt ORP electrode/reference electrode and a data-logger. The protocol offers significant improvements over the commonly used methods and is producing accurate and reliable reading of ORP in soils and water. By directly measuring the ORP and recording its changes overtime it was possible to get a true measure of the redox conditions in the soil system. It appears that the main source for unreliable results reported earlier when platinum electrodes were used, is the presence of minute quantities of oxygen in the vicinity of the electrode surface added to the system when inserting the electrode into soil or water. In order to obtain reliable measurements when using an ORP electrode it is necessary to wait for equilibrium reached after the disturbance created by inserting the electrode has subsided. It should be emphasized that equilibrium is expected to be reached after a long period (at least few hours for aqueous systems and few days for soils), rather than as commonly done where readings are taken few minutes to few hours after insertion of the electrodes. This is particularly important in reduced systems, since it takes quite a while for the microbial population and the diffusion processes to consume/diffuse the air oxygen added by the insertion of the electrodes. The results of this study show that long-term measurement and recording of the ORP is both feasible and necessary for obtaining reliable characterization of the true redox conditions in environmental systems.
ACKNOWLEDGMENTS This project was part of an extended study of heavy metal and trace element adsorption to recharge basin soils of Shafdan reclamation project and was
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partially supported by Mekorot Water Co., Israel and by a fellowship of the Hebrew University to G. Eshel. We would like to acknowledge the useful help and cooperation of the Shafdan operation crew, especially of N. Icekson-Tal, R. Blank, and G. Shoham. We are also grateful to D. Broker from Fourier Systems Co., Israel for technical support. The field and laboratory help provided by Y. Yablekovich, Y. Shachar and, in particular, D. Greenwald from the Dept. of Soils and Water Sciences, is gratefully acknowledged.
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