Variability of total and dissolved elements in stormwater runoff.
ABSTRACT: Stormwater runoff may contribute to groundwater pollution. This study evaluated seasonal influence on runoff water quality and compared urban runoff water quality with U.S. Environmental Protection Agency (EPA) drinking water standards. Stormwater from sixteen playa lakes were evaluated for total and dissolved elements during a 32-month period. Twenty elements were monitored to determine if concentrations posed a potential groundwater pollution problem. Eleven of the elements evaluated were temporally correlated with season of the year for both total and dissolved concentrations. Seasonal significances of the total and dissolved elemental concentrations were explained by natural or anthropogenic causes. The majority of the elements considered hazardous to human health, appeared in low concentrations in these urban storm water fed lake waters during the study period Concentrations of total and dissolved Al and Fe exceeded the U.S. EPA's secondary drinking water standards (cosmetic and aesthetic, not health). No significant health risk would seem to be posed by this urban stormwater runoff percolating to groundwater.
Keywords: Playa lakes, primary drinking water regulations, secondary drinking water regulations, urban stormwater runoff
The Southern High Plains (SHP) region of New Mexico and Texas has nearly level to gently undulating topography interrupted by numerous enclosed depressions. Playas are ephemeral lakes that occur in the bottoms of these depressions. There are approximately 30,000 playa lakes in the SHP (Osterkamp and Wood 1987). In Lubbock, Texas, and other SHP cities, playas serve as catchment basins for stormwater runoff. Stormwater runoff containing pollutants from streets, residential areas, and commercial enterprises collect in these playa lakes. The playas store runoff until the water evaporates or infiltrates into the soil to recharge the groundwater.
Urbanization increases areas of impermeable surfaces on the contributing playa watersheds. Impermeability results, not only in a greater frequency of runoff events, but also increased volumes of runoff generated during precipitation events. Playa lakes in Lubbock also have been modified by land developers and the city, which reduces their surface area and increases their lake depth. These modifications augment recharge of the ground waters in two ways. First, the Randall clays (fine, smectitic, thermic Ustic Epiaquerts) that underlie the playas are stripped away during the excavation. Removing the clay exposes more permeable material, enhancing percolation. Second, recharge is improved through increased hydrostatic head.
The quantity and quality of resulting urban playa lake waters are of special concern to the City of Lubbock and other SHP cities because of their multipurpose use. Urban areas of the SHP have rising groundwater levels from playa recharge caused by increased frequency and volumes of runoff coupled with increased soil permeability and hydrostatic head (Zartman et al. 1994). Some urban playas have ceased being ephemeral lakes and have become permanent, "water table" lakes. These lakes, scattered throughout the urban areas of the SHP, are valued for recreational fishing and boating in an area noted for its shortage of lakes and rivers. The lakes also provide a natural habitat for wintering migratory waterfowl (Bolen et al. 1989).
From this perspective, concentrations of pollutants that enter the playa lake via stormwater runoff are of interest in assessing the value of lake water for recreational and other potential uses, as well as for subsequent impacts on the underlying groundwater (U.S. EPA 1995). Heavy metal concentrations in the playa lakes are a concern because these elements can seriously affect human populations and aquatic organisms (Elder 1988). Transferred through food chains or transported to potable water sources, metals can create special problems for human health and aquatic resource management.
The link between lake water quality and the underlying Ogallala Aquifer became apparent in the late 1970s and early 1980s because of the rising groundwater levels in portions of the city. These findings caused a reevaluation of the significance of playas throughout the whole SHP region (Zartman et al. 1994, 1996).
Playa basin-to-basin water interchange happens only during the most highly unusual (25 yr) runoff events. Absence of playa basin-to-basin water transfer provided us with an opportunity to compare water quality in lakes following urban stormwater runoff events.
The overall objectives of this study were to evaluate dissolved and total element concentration in 16 playa lakes resulting from urban stormwater runoff in Lubbock, Texas. The specific objectives of the study were: 1.) to compare elemental concentrations of runoff water in the lakes with the National Primary and Secondary Drinking Water Regulations as established by the U.S. EPA; and 2.) evaluate elemental concentrations in lake waters by the season in which the runoff occurred.
Materials and Methods
We evaluated 16 urban playa lakes in Lubbock, Texas, for stormwater runoff quality. This 32-month experiment began in December of 1991. We collected water samples for total and dissolved elemental analysis from each playa the first day after stormwater runoff occurred. Grab samples were collected at depths 50-200 mm beneath the water surface at a sampling point closest to an adjacent street. Number of playas times number of rainfall events were considered the replications. For the total elemental analyses, waters from eight runoff events (128 replicates) were evaluated. Waters from these eight events plus an additional nine events (272 replicates) were analyzed for the dissolved elements. For total element analysis, we used the unfiltered water containing suspended solids as well as dissolved inorganics. We filtered the water samples using a 25 [mu]m filter before dissolved element analyses. Aluminum (Al), arsenic (As), barium (Ba), cadmium (Cd), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), mercury (Hg) [total analysis only], molybdenum (Mo), potassium (K), sodium (Na), selenium (Se), silver (Ag), vanadium (V), and zinc (Zn) were evaluated using atomic absorption spectrometer and graphite tube atomizer methods (U.S. EPA 1979). Strontium (Sr) was analyzed using the method developed by Clesceri et al. (1989).
We grouped sampling dates based on Lubbock's seasonal precipitation patterns. The mean annual precipitation in Lubbock is 465 mm (NOAA 1997), with most runoff occurring from April to October because of thunderstorm activity. Light rain or snow from low pressure storms produced runoff in winter (December, January, and February). Spring (March, April, and May) and summer (June, July, August, and September) runoff comes primarily from thundershower activity associated either with the passage of fast moving frontal systems (spring) or from convective heating (summer). Fall weather (October and November) runoff events are associated with monsoonal weather patterns flowing from the southwest. We analyzed total and dissolved elements as a function of season (treatments) using analyses of variance (SAS 1985). Duncan's multiple range test was used to separate specific differences at the 5% level (Pr = 0.05) in the treatment (season) means.
Results and Discussion
Stormwater runoff vs. drinking water. Summaries of the results for the 20 total element concentrations and the 19 dissolved element concentrations evaluated are presented in Table 1. This table shows maximum and minimum concentrations, average value, and geometric mean for the total and dissolved metals evaluated. Apparent inconsistencies in the maximum, arithmetic, and geometric means (Table 1) are explained by the increased sample numbers for dissolved element analyses. Increased funding allowed for greater sample numbers of dissolved element analyses and these maximum extremes occurred in the additional samples. We compared the water quality from stormwater events with maximum contaminant levels (MCLs) established by U.S. EPA for primary (U.S. EPA 1997) and secondary water drinking water regulations (U.S. EPA 1996). The National Primary Drinking Water Regulations are associated with potential health effects from ingestion of water. The National Secondary Drinking Water Regulations are "non-enforceable guid elines regulating contaminants that may cause cosmetic...or aesthetic effects" (U.S. EPA 1997). Although these playa lakes capture and store stormwater runoff, which may percolate and recharge the underlying groundwater, playa lakes per se are not drinking water sources. Many urban areas use groundwater for municipal drinking water; therefore, the U.S. EPA standards were used as comparison benchmarks.
No uniform pattern characterizes the elemental concentrations and the U.S. EPA's MCLs. Some of the elements that we evaluated (K, Ca, Mg, and Na) are not included in the U.S. EPA guidelines. Concentrations of total and dissolved Al and Fe (Table 1) exceeded U.S. EPA's Secondary Drinking Water Regulations. These non-attainments of regulation values are cosmetic or aesthetic problems rather than detrimental health effects. Values for the average and geometric mean for the remainder of the total and dissolved metals were below the U.S. EPA's MCLs. At times, however, individual sample values for total metal playa lake waters of Cd, Hg, and Pb exceeded the U.S. EPA's MCLs for potable water supplies. One of the 128 samples for total Cd concentration exceeded the MCL. Total concentration of Hg and Pb exceeded the MCLs in 42 and 23 of the 128 samples, respectively. While two of the 272 dissolved sample values for Pb exceeded the MCL value; none of the other dissolved element concentrations exceeded their MCLs.
Seasonal considerations. Stormwater runoff events were grouped by season for temporal analysis. Dissimilarities were noted also in the elemental patterns of total playa lake water concentrations with season. Total concentrations of Ag, Cu, Fe, K, Mn, and Se were not significantly different by the season. Total concentrations of Al, As, Ba, Ca, Cd, Cr, Hg, Mg, Mo, Na, Pb, Sr, V, and Zn in the playa lakes differed significantly with the season of the year (Table 2). We attributed those differences in total elemental concentration to natural (Al, Ca, Mg, Sr, Hg, Mo, V) or anthropogenic (As, Pb, Na, Zn, Ba, Cd, Cr) causes.
Large differences in the total concentrations of Al, Ca, and Mg (Table 2) are related to high eolian sediment accumulations during the spring. Temporal distribution patterns of the eolian transported Ca showed low total concentration in the fall when eolian accumulations are the least and higher concentrations during the other seasons. Aluminum and Mg, being components of the octahedral sheet of 2:1 clays, were associated with eolian clay movement. Intermediate quantities of Mg were entrained in the stormwater sediments in the fall and winter seasons with the least in the summer (Table 2). Higher total concentrations in winter, spring, and summer reflect increased eolian activity.
Other differences in natural causes are related to soil conditions (Mo) or plant growth activity (Hg and V) during the spring and summer. Total concentrations of Mo were determined to be in highest quantities in the summer season (Table 2). We believe that the higher concentration was due to the chemical characteristic of being more soluble under high pH conditions (Mortvedt et al. 1991). Kabata-Pendias and Pendias (1992) reported Mo was most soluble in "wet alkaline soils" which are present in Lubbock playas during the summer season. Total Hg concentrations showed lowest concentrations in the fall but increased as the plant-growing season lengthened. Absorption of Hg by humic substances in water and soil particles has been reported to be high (Crock 1996), and Hg concentration parallels organic matter accumulation in playa lakes. Vanadium is associated also with organic matter and there are significantly greater quantities of total V concentration (Table 2) in the playas when the organic matter is actively b eing produced (summer) than during the other seasons.
We postulate anthropogenic differences related to herbicides, automobiles, fertilizers, and ice removal. The anthropogenic element associated with herbicide is As. The As data showed the total concentrations for the playas to be high in the spring of the year (Table 2). Arsenic data showed highest total concentration in the spring and lowest total concentrations in the summer. Arsenic has long been used as a cotton herbicide and defoliant as well as a herbicide for weed control in lawns (Klingman and Ashton 1975). High As concentrations probably were caused by the influx of eolian materials during the spring dust-storm season. Dust storms are quite prevalent in the spring, and the absorbed As entrained in eolian sediments are transported to playas with stormwater runoff. The element associated with automobile use, Pb, was significantly higher in total concentrations during the summer and fall seasons compared to winter and spring periods. Automobile emissions were a main source of Pb pollution as halide salts (Moore et al. 1977). Total Pb concentrations (Table 2) exhibited higher levels in summer and fall periods that normally exhibit a smaller frequency of rainfall events in the SHP than do late winter and spring. Infrequent runoff events allow for greater entrained Pb concentrations. While Na is not directly associated with automobiles, the city of Lubbock uses salt to de-ice roadways for automobile use. Na was significantly higher in the spring because of runoff from parking lots, sidewalks, and streets in the city when salt was applied.
Seasonal distributions of total element concentrations of other elements (Sr, Ba, Cd, Cr, and Zn) in Lubbock playa lake waters are less well explained. The total elemental concentration of Sr is the least (Table 2) in the spring of the year when eolian accumulations are the greatest. Since Sr competes with Ca for clay adsorption sites, we postulate that Sr is low in concentration due to the very large influx of CaCO3 and clay. The anthropogenic elements associated with fertilizer use are Ba, Cd, Cr, and Zn. Barium, Cd, and Cr, while not added directly, are impurities of P fertilizer (Kabata-Pendias and Pendias 1992) that is applied in the fall. All three of these elements are significantly greater in total concentration in the fall (Table 2). Zinc fertilizer, foliarly applied in the summer, had greater total concentrations in the summer and fall runoff events (Table 2).
Of the 19 dissolved element concentrations evaluated in the Lubbock playa lalces, Ag, Cr, Cu, Fe, Se, and Sr concentrations were not significantly different with season. We noted significant seasonal concentration differences for the dissolved elements Al, As, Ba, Ca, Cd, K, Mg, Mn, Mo, Na, Pb, V, and Zn (Table 3). As in the case with the total element concentration of the playa lake waters, Al, As, Ca, and Mg, were greatest in spring due to eolian processes. High Ba, K, Mg, V, and Zn concentrations (dissolved elements) for spring and early summer (Table 3) were the results of increased fertilizer entering the playa lakes. Increased spring and early summer runoff amounts from a higher frequency of precipitation events and low vegetal cover probably caused increased inflow of suspended solids. Total Ba concentrations were high in the fall (Table 2) while dissolved was high in the spring (Table 3). Our explanation is that Ba, as well as Cd, are impurities in phosphorus fertilizer that is typically applied in th e fall to promote root growth. While P is relatively immobile in the soil, in urban environments much applied fertilizer lands on such impermeable surfaces as sidewalks and driveways and becomes entrained in the stormwater runoff. In this case, Ba and Cd concentrations are high in total amounts determined for the lakes in the fall. Specific statistical significant differences for Cd were identical for the dissolved and total concentrations. These differences are possibly due to additions as fertilizer impurities. Potassium fertilizer is reflected in the high total K content of the lakes during the spring. Molybdenum and V are associated with the organic matter content of the lakes, are high in both total, and dissolved quantities in the winter and spring. By autumn, these elements have been assimilated into plant material and have low concentrations within the water. Total Na is high in the spring reflecting its use as a deicing agent, but the occurrences of high fall dissolved Na concentrations are unexplain ed.
As in the case of the total elemental concentration in the playa lakes, certain elements cannot be definitely explained. The elements Mn and Pb are in this class. Dissolved Mn concentrations were significantly greater in the summer than in the fall or spring (Table 3). Winter dissolved Mn concentrations were not significantly different from any other season. Dissolved Pb concentrations were significantly greater in the fall more than any other season, while the Pb concentrations in the spring were significantly lower than either winter or summer (Table 3).
Summary and Conclusion
Stormwater runoff from contributing watersheds exhibited seasonally variable total and dissolved metal concentrations in the 16 urban playas evaluated. Fourteen elements (Al, As, Ba, Ca, Cd, Cr, Hg, Mg, Mo, Na, Pb, Sr, V, and Zn) examined over eight sampling events for the 16 playas showed significant total concentration variations with season. Thirteen elements (Al, As, Ba, Ca, Cd, K, Mg, Mn, Mo, Na, Pb, V, and Zn) examined over 17 sampling events, showed significant dissolved concentration differences with time. In the Lubbock playa lakes, eleven of the 20 element concentrations evaluated (Al, As, Ba, Ca, Cd, Mg, Mo, Na, Pb, V and Zn) were temporally associated with season of the year for both total (Table 2) and dissolved concentrations (Table 3). Only Cd had an identical statistical distribution pattern for total and dissolved concentrations. Eolian deposition and fertilizer practices are postulated to explain the seasonally significant differences in the dissolved and total elemental concentrations. Wind erosion, that formed the SHP, is responsible for the elements--Al, As, Ca, Mg--entrained in the playa water. Soil conditions (Mo) or plant growth activities (Hg and V) during the spring and summer are postulated to be responsible for other seasonal distributions. Anthropogenic causes of the elemental changes in the stormwater runoff are due to automobiles (Pb), fertilizers (Ba, Cd, Mg, Zn), or deicing salt (Na).
The playa lake stormwater runoffs that we evaluated are not drinking water sources. Our comparisons of stormwater runoff water to the benchmark concentrations of U.S. EPA's maximum contaminant levels (MCLs) were for illustrative purposes only. Total and dissolved Al and Fe concentrations exceeded the U.S. EPA's Secondary Drinking Water Regulations, but are not thought to pose human health problems. The MCLs for total Hg and Pb were exceeded 42 and 23 times, respectively. Geometric mean values of Hg and Pb, however, did not exceed the U.S. EPA's MCLs. Two dissolved metal samples in the 272 samples analyzed for Pb exceeded the U.S. EPA's MCL value. Therefore, the stormwater runoff and the entrained elements would seem to pose no apparent health hazard to human or aquatic life.
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Table 1 Total and dissolved element concentrations for playa lake waters from 16 urban playa lakes in Lubock, TX. Total Elements MIN Value MAX Value Arithmean Geomean Metal mg [L.sup.-1] mg [L.sup.-1] mg [L.sup.-1] mg [L.sup.-1] Ag 0.001 0.008 0.001 0.001 Al 0.052 20.8 1.72 0.877 As 0.001 0.026 0.006 0.006 Ba 0.002 0.768 0.179 0.135 Ca 4.00 118 39.3 36.1 Cd 0.000 0.006 0.001 0.000 Cr 0.001 0.042 0.023 0.008 Cu 0.001 0.172 0.003 0.0019 Fe 0.087 24.1 1.70 0.957 Hg 0.001 0.078 0.004 0.002 K 1.00 23.2 7.58 6.42 Mg 1.32 32.8 6.49 5.16 Mn 0.002 0.320 0.0393 0.026 Mo 0.001 0.120 0.013 0.008 Na 4.38 171 46.4 35.3 Pb 0.001 0.089 0.011 0.007 Se 0.002 0.004 0.002 0.002 Sr 0.004 1.51 0.481 0.370 V 0.004 0.123 0.018 0.011 Zn 0.002 0.366 0.055 0.040 Dissolved Elements MIN Value MAX Value Arithmean Geomean Metal mg [L.sup.-1] mg [L.sup.-1] mg [L.sup.-1] mg [L.sup.-1] Ag 0.001 0.002 0.001 0.001 Al 0.048 63.6 2.86 0.976 As 0.001 0.030 0.006 0.005 Ba 0.002 0.510 0.163 0.136 Ca 4.80 149 29.1 27.3 Cd 0.000 0.002 0.000 0.000 Cr 0.001 0.018 0.002 0.001 Cu 0.001 0.074 0.009 0.004 Fe 0.025 4.82 0.629 0.410 Hg K 0.360 14.2 5.03 4.48 Mg 1.00 17.5 3.71 2.99 Mn 0.001 0.147 0.021 0.0133 Mo 0.001 0.199 0.010 0.004 Na 1.10 79.00 18.0 13.6 Pb 0.001 0.022 0.003 0.002 Se 0.002 0.003 0.002 0.002 Sr 0.003 1.44 0.400 0.323 V 0.004 0.082 0.011 0.008 Zn 0.001 0.097 0.022 0.021 Table 2 Arithmetic mean total elemental concentration by season for 16 urban playa lakes in Lubbock, TX. Season Element Fall Winter Spring Summer (2 Events) (2 Events) (1 Event) (3 Events) Mg [L.sup.-1] Al 0.867 b (*) 1.79 a 1.36 ab 1.98 a As 0.00538 b 0.00628 b 0.0129 a 0.00510 b Ba 0.279 a 0.164 bc 0.200 b 0.117 c Ca 29.3 c 38.5 b 54.1 a 41.6 b Cd 0.00103 a 0.000273 b 0.000136 b 0.000519 b Cr 0.00889 a 0.00390 b 0.00610 ab 0.00447 b Hg 0.00190 b 0.00229 ab 0.00222 ab 0.00630 a Mg 7.31 b 5.80 bc 10.0 a 4.60 c Mo 0.0100 b 0.00585 b 0.00647 b 0.0230 a Na 4.88 b 4.07 b 73.4 a 39.6 b Pb 0.0132 a 0.00688 b 0.00369 b 0.0122 a Sr 0.584 a 0.489 a 0.151 b 0.513 a V 0.0101 b 0.00993 b 0.0160 b 0.0273 a Zn 0.0721 a 0.0444 b 0.0396 b 0.0571 ab (*)Similar letters within the same different (Pr < 0.05) from one another. Table 3 Arithmetic mean dissolved element concentration by season for 16 urban playas in Lubbock, TX. Season Element Fall Winter Spring Summer (3 Events) (5 Events) (4 Events) (5 Events) Mg [L.sup.-1] Al 1.06 b (*) 2.97 ab 3.92 a 2.97 ab As 0.00514b 0.00653 a 0.00601a 0.00465 b Ba 0.161 b 0.163 b 0.198 a 0.139 b Ca 26.6 b 30.7 a 34.7 a 24.4 b Cd 0.00275a 0.00125b 0.00140b 0.00153b K 4.23 b 4.60 b 5.63 a 5.43 a Mg 4.05 ab 3.60 bc 4.90 a 2.85 c Mn 0.0261 a 0.0201 ab 0.0251 a 0.0157 b Mo 0.00292 b 0.00659 a 0.00423 ab 0.00672 a Na 23.5 a 21.2 ab 11.9 c 17.7 b Pb 0.00404 a 0.00303 b 0.00203 c 0.00305 b V 0.00576 b 0.00965 ab 0.0132 a 0.0131 a Zn 0.0183 b 0.0171 b 0.0270 a 0.0237 a (*)Similar letters within the same row are not significantly different (Pr < 0.05) from one another.
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|Author:||Zartman, R.E.; Ramsey, R.H. III; Huang, A.|
|Publication:||Journal of Soil and Water Conservation|
|Article Type:||Statistical Data Included|
|Date:||Jun 22, 2001|
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