Spatial variability of solutes in a pecan orchard
surface-irrigated with untreated effluents in
the upper Rio Grande River basin
N.W. Assadian
a,*, L.B. Fenn
a, M.A. Flores-Ortiz
b, A.S. Ali
a aDepartment of Soil and Water Science, Research Center, Texas Agric. Exp. Sta.,Texas A&M University, 1380 A&M Circle, El Paso TX, 79927, USA
bInstituto Nal. De Investigaciones Forestales Y Agropecuarias Campo Experimental,
Valle De Juarez, KM. 65 Carretera, Juarez-Porvenir, Praxedis, G. Gro. Chih, Mexico
Accepted 15 February 1999
Abstract
Untreated effluents are blended with water from the Rio Grande River and used for irrigation in the Juarez Valley of northern Mexico. Effluents are a source of nutrients, but may also be a source of heavy metal contamination. This study was conducted to characterize deposition patterns of
selected metals, salts, and total nitrogen in a 6 ha pecan (Carya illinoenisisK.) orchard which had
healthy-to-stunted trees with dieback. Orchard soil was collected along multiple transects to depths of 1.2 m, with spacing every 20 m. All solutes showed a magnitude variability in particular ions.
Chromium, Ni, Pb, and Cd concentrations averaged <14 mg kgÿ1. Soil Na, Ca, K, Mg, SO4, Cl and
NO3±N averaged <100 mg kgÿ
1
. Total N was <0.21%. Most solutes accumulated at the soil surface
with the exception of Na and SO4. Linear semi-variograms best described spatial metal deposition
and surface clay content with a range of influence >189 m. Spherical semi-variograms best described spatial distribution of salts and total N, but accounted <50% of the variability. The solubility of solutes in moderately alkaline irrigation water and their specific behavior in calcareous soils likely affected deposition patterns. Estimated metal loads from irrigation over a 15-year period
were <3 kg haÿ1, but about 187 Mg haÿ1for total dissolved solids (salts). Pecan leaf tissue showed
no signs of heavy metal accumulation. Suboptimum pecan growth was associated with salt accumulation in a clayey area with low permeability. Salts, in particular Na, rather than metals may be the most important inorganic contaminants for irrigated agriculture in this region. Salt loads in irrigation waters are expected to increase as agriculture increasingly relies on urban effluents too
expensive to convert to potable water.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Spatial variability; Heavy metals; Salts; Wastewater; Irrigation; Pecan orchard; Calcareous soils Agricultural Water Management 42 (1999) 143±156
* Corresponding author. Tel.: +1-915-859-1908, ext. 12; fax: +1-915-859-1078
E-mail address:n-assadian@tamu.edu (N.W. Assadian)
1. Introduction
The fate and transport of solutes in field soils is frequently modeled to improve our understanding and management of field-applied chemicals. Of increasing importance is the transport of solutes introduced by irrigation waters of degraded quality. Snow et al. (1994) found that hydraulic processes at the soil surface, small-scale heterogeneity, and field-scale variation significantly interfered with attempts to model solute transport under actual field conditions. Miyamoto and Cruz (1986) concluded that field-scale variability could be reduced in the appraisal of salinity and sodicity in surface-irrigated pecan (Carya illinoensisK.) orchards if soil samples were grouped according to soil mapping units to a depth of 0.6 m. Cahn et al. (1994) demonstrated that the range of spatial dependence of particular solutes depended on the unique chemical properties of solutes and their behavior in soil.
Along the international border between Mexico and Texas, water from the Rio Grande River is the dominant irrigation source. Both treated and untreated sewage effluents are often discharged into lateral irrigation canals or mixed with river water and used for surface irrigation. Recent assessments of water quality in the upper reach of the Rio Grande River have found nitrogen (N), phosphorus (P), chlorine, salts, and heavy metals to be at levels of concern (IBWC, 1994). Untreated effluents on the Mexican side of the border constitute about 40% of the irrigation source.
Farmers in the border region contend that salt and heavy metal loads in river water have increased as population and industrialization has escalated over the past 40 years. There are limited historical data on local water quality and soil chemistry to evaluate the current or future impacts of these irrigation waters on field soils, particularly in Mexico. Understanding the movement and fate of different solutes in wastewaters used for irrigation is of increasing importance, particularly near urban centers. Our objective was to utilize geostatistics as a tool to characterize deposition patterns of selected metals, salts, and nitrogen in a pecan orchard soil. Solute accumulation associated with irrigation entry would implicate the role of irrigation as a primary contamination source.
2. Procedure
A 6 ha pecan orchard located about 20 km south of the El Paso, Texas/Cuidad Juarez, Mexico metroplex in San Isidro, Chihuahua was the selected experimental site (Fig. 1). The orchard was fairly representative of 300 ha of pecans in the Juarez Valley that constitute about 1.3% of the total agricultural land. All crops in this arid region (rainfall <254 mm) are strictly dependent on irrigation. Orchards are irrigated with a combination of water from the Rio Grande River, aguas negras (untreated sewage effluents), and moderately saline well water (>1000 mg kgÿ1 total dissolved solids). Raw sewage constitutes approximately 40% of the irrigation water during the growing season. Sewage effluent is the dominant water source after September.
Rio Grande River water originates from watersheds in southern Colorado and northern New Mexico, USA, and stored in a reservoir above Elephant Butte Dam, NM, with a design capacity of 3.25 billion m3(Miyamoto et al., 1995). A flow of 547 million m3
Fig. 1. The pecan orchard in Mexico selected for extensive soil sampling to a depth of 1.3 m.
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enters West Texas with 65 million m3diverted to Mexico. Nutrient and salt loads in river water are well documented on the Texas side of the border before the flow is diverted to Mexico at the International Dam (Table 1). In the Juarez Valley in Mexico, historical data on river quality data is sketchy, and heavy metal concentrations are limited for the upper reach of the Rio Grande River.
Orchard soils were calcareous Typic Torrifluvents. These are level, deep, brown soils that developed in loamy, friable sediments recently deposited on the flood plain of the Rio Grande. Soil preparation prior to pecan planting included plowing to a depth of 0.3 m, disking, and then leveling operations. The orchard floor was planted with Bermudagrass (Cynodon dactylonL.), with limited soil disturbance after orchard establishment. Cattle (<25 head) had been allowed to graze on the grass.
The orchard had an east/west orientation with regard to the irrigation canal (Fig. 1). Orchard dimension was 200300 m2with trees spaced every 10 m. Trees were about 15 years old and of variable size. The sampling grid was composed of three parallel transects spaced at least 60 m apart in an east/west direction, perpendicular to the irrigation canal. A fourth transect was established in the middle of the orchard in a north/south direction (Fig. 1). Soil cores were collected to a depth of 1.2 m every 20 m on each transect with three additional sites spaced 10 m apart on the fourth transect (Fig. 1). There were a total of 42 soil cores that were partitioned into depth segments of 0±0.5, 0.5±0.3, 0.3±0.6, 0.6± 0.9, and 0.9±1.2 m. Abrupt changes in soil texture within each 0.3 m segment were partitioned and stored separately.
Partitioned soil samples were air-dried and particle size distribution (texture) determined using the hydrometer method (Black et al., 1965). Homogenous subsamples were run through a 2 mm sieve and/or mechanically ground then pulverized using a SPEX Mixer/Mill. A 1 : 1 soil-to-water extract was obtained for routine analyses of pH and soil salinity (electrical conductivity; EC). Soil solutes were grouped as metals,
Table 1
Concentration ranges of solutes found in Rio Grande River water in Texas, US, before diversion to Mexico and after it has been blended with untreated sewage effluent for irrigation
Parameters Rio Grande River before the
International Dam, Texas (1988±1990)a
bUnpublished data from the Frank Hernandez Laboratory at the Texas A&M Res. Ctr., El Paso, Texas.
cations, anions, and N forms. Heavy metals were determined from acid digestion of pulverized subsamples following USEPA protocol 200.2 (USEPA, 1991). Chromium, Ni, Pb, and Cd were determined using the ICP technique USEPA protocol 200.7 (USEPA, 1991). Method detection limits (MDL) were determined using EPA 40CFR Part 136, Appendix B. Soluble cations and anions (Na, K, Ca, Mg, Cl, NO3, SO4) were determined
from 1 : 1 water extracts. Microsamples of soil were combusted to determine total N using a NCHS-O analyzer.
A total of 18 composite pecan leaf samples were collected in duplicate from at least three trees within grids delineated by Transects 1, 2, and 3 (across the fourth transect). Leaves were washed in a 0.1 N HCl and double-rinsed with de-ionized water and dried in a forced-air dryer at 658C for at least 72 h. Plant tissue was ground and passed through a 2 mm sieve. Sub-samples were acid-digested using a combination of nitric acid (pre-digestion) and perchloric acid. Chromium, Ni, Pb, Cd, Na, Ca, K, B, and Mg were determined using ICP technique (USEPA 200.7). Plant tissue was combusted using an NCHS-O analyzer to determine total N.
Solutes in soil and pecan leaf were analyzed for mean, extremes, standard deviation, and coefficients of variability, and correlated with soil texture, visual estimates of tree size, and distance from the irrigation canal. Assuming spatial independence, classical analysis of variance as a one-way layout was computed to detect differences of solutes by transect, depth, or the percentage of clay content. Normality of the data was examined by kurtosis and skewness. If skewness coefficients exceeded 1.0 then outlying data greater than four standard deviations from the mean were discarded or the entire dataset transformed to natural logarithms to stabilize variance (Cahn et al., 1994 and Gamma Design Software, 1992). Isotropic semi-variance of data was calculated to determine if there was spatial dependence of solutes with regard to irrigation delivery using the following equation:
where is the semi-variance for n data pairs separated by a distance of h and Z is
the value (in this case metal concentration) at positions xi and xih (Clark, 1979 and Warrick et al., 1986). Lag classes of 20 m were chosen up to a distance of at least 108 m.
Four idealized semi-variogram models are illustrated in Fig. 2. The first is simply a random model, that is, there is no spatial interdependence and the semi-variogram function(h) is simply a horizontal line. The second has a finite maximum or sillCand a
random component expressed as the nuggetCo. There is range `a' which is the separation
distance for which the function approaches the sill. The third and fourth models have no sill and hence the variance tends to increase according to the length over which it is evaluated. For the linear model the value increases from a nuggetCo(which can be zero
or positive) and increases linearly without reaching a maximum; for the parabolic model (the fourth one shown) the increase inis faster than for the linear case. The parabolic
square analysis rather than by more sophisticated analyses which would be appropriate to remove drift if larger sample numbers were available.
3. Results
3.1. Soil solute concentration and variability
A clay loam dominated surface soil to a depth of 0.3 m underlain by loam to 0.6 m followed by layers of sandy loam or loamy sand. Sandy subsurface strata indicated that irrigation drainage was not restricted in this orchard. At the soil surface, clay content was greatest in the northwest corner of the orchard next to the irrigation canal where Transect 1 was located (Fig. 1). Soil textures became coarser with distance in both the east and south directions where Transect 3 was located.
Maximum metal concentrations were <33 mg kgÿ1 and averaged <14 mg kgÿ1 (Table 2). Relative metal concentrations were in the order Cr > Ni > Pb > Cd. Cation and anion concentrations were often >200 mg kgÿ1and averaged <100 mg kgÿ1. Sodium levels dominated cation concentrations by at least two-fold and relative concentrations were Na > Ca > K > Mg. Relative anion concentrations were SO4> Cl > NO3. Additional
measurement of soil solution showed EC infrequently exceeded 2.5 dS mÿ1and averaged 0.4 dS mÿ1. Sodium absorption ratios ranged from 3.4 to 4.3 among soil depths indicating a non-saline, non-sodic soil (USDA, 1954). However, salinity and sodicity measurements were not based on a saturation paste, but rather a 1 : 1 water-to-soil extract. This could
Fig. 2. Shapes of semi-variograms. In the sperical model,Csample variance,Coa random component,
CCothe sill, and a denotes the range of sample influence of distanceh.
mean that salinity may be two-fold greater than our measurements suggested, particularly on sandy textured soils. Total N was <0.21% in orchard soil samples which is typical in mineral soils of the western US.
Solutes showed a magnitude of soil concentrations (Table 2) and coefficients of variability (CV) ranged from 34 to more than 300%. Metals were least variable and cations most variable. Surface metal variability within the horizontal soil plane of the pecan orchard showed a negative correlation with increasing distance from irrigation delivery except for Pb. Vertical depositions of all solutes except Na and SO4significantly
decreased with increasing soil depth. Solute accumulation highly correlated with increasing clay content. Increasing solute concentrations were associated with smaller pecan tree size with the exception of K and N forms. The concentration of one soil metal also correlated with those of other soil metals (data not shown). The same was found among ions and N forms.
Selected solutes in Tables 3 and 4 represent general results from traditional ANOVA analyses. Metals showed significant differences among transects, depths, and soils with increasing clay content. These results supported correlation findings (Table 2). Between and within (error) mean square values for cations and anions were more similar than those for metals and as a consequence significant deposition patterns were not as clear cut. Sodium concentrations only differed between transects and clay content and generally reflected ANOVA results for those of Ca, Mg, and SO4. Potassium differed only by depth Table 2
Descriptive statistics of metals, cations, anions, and nitrogen from soil samples collected to a depth of 1.3 m in a surface-irrigated pecan orchard and correlation coefficients (r) of each solute with distance (d), depth, percentage clay content, and pecan tree size
Solute Concentration (mg lÿ1) r
Min Max Avg. SD CV (%) d Depth Clay content Tree size
Metals
and Cl varied only between transects. These results were not always congruous with correlations in Table 2. Nitrate±N and total N differed between soil depths and clay content and supported correlation findings.
A look at mean solute values suggested that metal accumulation was highest in the northwest side of the field (Transect 1), at the soil surface to a depth of 0.3 m, and in high
Table 3
ANOVA for transects, soil depth, and clay content for selected solutes collected from a surface-irrigated pecan orchard
aSignificant at 0.05% level of probability. bSignificant at 0.01% level of probability.
Table 4
Mean values of selected solute concentrations by transect, depth, and the percentage clay content in soil from a surface-irrigated pecan orchard
Obs. Cr Pb Na K Cl NO3-N TN (%)
mg kgÿ1
Transect
1 42 17.5aa 8.2a 67ab 8NS 39a 20NS 0.06NS
2 39 11.5bc 6.7b 58ab 20 31ab 19 0.05
3 43 10.3c 5.9b 47b 6 26b 11 0.05
4 74 13.2b 6.7b 81a 8 43a 10 0.06
Soil depth
0±0.03 39 14.5a 9.1a 58NS 24a 26NS 34.5a 0.12a
0.03±0.3 40 14.6a 8.6a 67 7b 33 17.6b 0.06b
0.3±0.6 39 13.8ab 6.2b 77 6b 44 11.5b 0.03c
0.6±1.0 40 11.8bc 5.5bc 63 6b 39 7.1b 0.03c
1.0±1.3 40 11.1c 4.7c 65 6b 37 6.6b 0.02c
Clay content
>60% 7 26.1a 9.8a 49a 7NS 46NS 28a 0.06ab
40±60% 19 20.8b 9.1a 49a 7 48 25a 0.08a
20±40% 62 14.3c 8.4a 38ab 17 37 22a 0.08a
<20% 110 10.4d 5.3b 23b 7 33 8b 0.03b
aNumbers followed by different lower case letters in each subcolumn are significantly different at the 0.05
level of probability. NS, not significant.
clay content (>60%) soil. Metals levels decreased by about 2 mg kgÿ1with every 0.3 m increment in soil depth. Soluble cations were more concentrated across the middle (Transect 4) of the pecan orchard in the 0.3±0.6 m subsoil layer containing 20±60% clay. Anions showed similar deposition patterns to cations, but were concentrated at depths below 0.3 m. Total N was uniform across transects at the soil surface and associated with soil textures having 20±60% clay content.
3.2. Spatial dependence
It is difficult to determine whether solute deposition patterns were related to distance, clay content, or the combination of both in this orchard. Plots of raw data along each transect added no visual clarification (Fig. 3). Isotropic semi-variance was conducted at soil levels where solute concentrations were greatest (Table 5). Linear models best described most metal deposition. Linearity of semi-variance suggested a gradual increase over the pecan orchard indicative of spatial interdependence. Spherical or exponential models best described Cd, cation, anion, and total N depositions. These models showed a more rapid change of variance over a limited distance until a horizontal sill was reached, indicating random variance. The range of spatial dependence for metals was >189 m except for Cd and >100 m for Cl and SO4. Spatial influence ranged from 32 to 98 m for
cations and35 m for NO3±N and total N.Covalues were generally low indicative of
low background noise of the data. However,r2values revealed that spatial dependence accounted for over 50% of the variance of surface metal accumulation, but accounted for
Table 5
Geostatistical models describing horizontal deposition of solutes and soil texture (expressed as the percentage clay content) from a surface-irrigated pecan orchard
Element Soil depth
(cm)
Transformation Model Co CoC a r2
Metals
Cr 0±0.03 none Linear 11 >48 >189 0.53
Ni 0±0.03 none Linear 3.3 >23 >189 0.54
Pb 0±0.03 none Linear 2.8 >18 >189 0.61
Cd 0±0.03 ln(Cd1) Exponential 0.001 0.1 55 0.51
Cations
Na 0.3±0.6 none Spherical 0.02 0.3 98 0.35
Ca 0.3±0.6 ln(Ca) Spherical 0.0001 0.2 47 0.33
K 0±0.03 ln(K1) Spherical <0.1 0.2 32 0.03
Mg 0.3±0.6 ln(Mg) Spherical 0.001 0.3 49 0.33
Anions
Cl 0.3±0.6 ln(Cl1) Spherical 0.2 1.5 122 0.42
SO4 0.3±0.6 none Linear 1555 >1555 >189 0.001
NO3±N 0.3±0.6 ln(NO31) Exponential 0.0 1.4 2.1 0.03
Nitrogen
Total N 0.3±0.6 none Spherical 0.0001 0.01 35 0.41
Soil texture
Clay (%) 0.3±0.6 none Linear 24 >168 >189 0.47
Fig. 3. Concentration distribution of clay content, chromium, sodium and nitrate nitrogen in a surface-irrigated pecan orchard located in Mexico.
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less than 50% of the variation of cations, anions, and total N (Table 5, Fig. 4). Miyamoto and Cruz (1986) recommended combing salt concentrations over two or more depths. However, this did not increase the accountability of variation (data not shown). Anisotropic semi-variance with directional rotation of the data also showed no improvement in semi-variance models (data not shown).
The range of spatial dependence generally agreed with those reported for metals (Assadian et al., in press), salts expressed as saturated conductivity (Warrick et al., 1986), and NO3±N (Cahn et al., 1994). However, these findings suggested that metals were the
only group of solutes that had deposition patterns related to space.
3.3. Pecan leaf analyses
There is always concern that contaminants in untreated effluents will be absorbed by plants. Soil metals generally had limited plant availability. Metal concentrations in pecan leaf tissue were within non-phytoxic ranges (Table 6). Average metal concentrations were at most 1.1 mg kgÿ1. Cadmium was detected in only one composite sample. Lead tissue concentrations tended to increase with Na tissue concentrations (p> 0.03), and were inversely related to Ni concentrations (p> 0.01) (data not shown). Cation and total N concentrations were also within conventional ranges. Tissue boron levels were also examined because it is often associated with reduced pecan growth and/or mortality (Picchioni et al., 1991). Tissue B concentrations ranged from 96 to 171 mg kgÿ1 and below reported levels of phytotoxity.
Fig. 4. Semi-variograms of soil clay content and selected solutes from a surface-irrigated pecan orchard located in Mexico.
4. Discussion
We would expect solute accumulation in untilled soil to be closest to the irrigation canal if irrigation water was the primary transport source. Water percolation and sediment deposition would be greatest near the water application point and decrease with increasing distance. Vertical movement would depend not only on the specific chemical and physical properties of each solute, but also that of the soil much like mobile and solid phases in chromatography. However, surface disturbance from pecan management tasks, grazing animals, or an uneven weed/grass cover could affect the irrigation wetting front and preferential flow.
An estimate of metal loading from irrigation was calculated assuming that the pecan orchard received about 1.3 m of water annually since orchard establishment 15 years ago. It was assumed that maximum metal concentrations in mixed river water were similar to those reported in Table 1. Calculated metal loads from irrigation were 2.9, 0.4, 0.75, and 0.18 kg haÿ1for Cr, Ni, Pb, and Cd, respectively. This would only account for at most 27% of the acid-available metals found in the top 0.03 m of the orchard floor. Prior to pecan establishment, the field was likely in row crop production which would account for additional metal loading from irrigation and other anthropogenic sources (Landrigan et al., 1975; Fergusson, 1990). Metals were likely transported in water and deposited in the orchard as precipitates. Non-point source inflow of metals into the irrigation network, regardless of initial solubility, would more than likely precipitate as a carbonate product, and become reasonably insoluble in slightly alkaline water. About 60±90% of the metals in the upper reach of the Rio Grande River are in the crystalline form (precipitated) in sediments and may not contribute to contamination with possible exceptions of Hg, Pb, and Cu (Miyamoto et al., 1995). Deposition of precipitated metals from irrigation would
Table 6
Descriptive statistics of minerals in pecan leaf tissue in comparison with conventional ranges
Element Pecan leaf concentration Conventional
Cd <MDL 0.3 0.02 0.07 350 0.1±7.6
%
follow Stoke's Law and would explain the wide range of spatial dependence described by the linear semi-variance model. The small fraction of unprecipitated metals should rapidly precipitate with soil carbonates. Clay content at the soil surface also increased metal accumulation. Permeability is more restrictive and clayey soils are more chemically reactive.
Unlike metals, cations and anions are ionized in irrigation water. Over a 15-year period, it was estimated that irrigation transported 187 Mg haÿ1of total dissolved salts into the orchard. Chloride and SO4account for 57 Mg haÿ1and the remaining 130 metric
tons were assumed to be primarily Na, Ca, and Mg. High solubility increased ion mobility. The type of ionic charge and valence affected the degree of soil mobility. Anions which are less reactive with soil had greater soil mobility than cations and leached to depths below 0.3 m. Dissolved Ca2 and Mg2 more readily precipitated with carbonates or adsorbed to soil exchange sites than Na or K. Similarly, SO4ÿ had
greater association with clay content, having anion exchange sites, than Clÿ. Ion mobility was, in general, inversely related to the range of spatial influence from semi-variance models and directly related to a random distribution (nugget effect).
Low NO3detection in irrigation water suggested that N was primarily transported into
the orchard as either NH4or organic forms. High surface accumulation of both NO3and
total N on the soil surface suggested that N was immobilized by microbes and converted to NO3 and/or cycled by the orchard ground cover (Roquette et al., 1973). We also
suspect that ground cover may be cycling soluble K.
Stunted pecan growth and tree mortality in the northwest corner of the pecan orchard appears to be due primarily to salt accumulation in clayey soil rather than metal contamination. Clayey soils under a minimum tillage system do not have sufficient permeability to leach salts (Miyamoto and Helmers, 1988). Underlying sandy soil may have also perched the water table, increasing salt accumulation within the effective root zone of the pecan trees. Pecans are moderately salt-tolerant and particularly sensitive to Na salts (Miyamoto and Helmers, 1988).
5. Conclusion
Salts, in particular Na, rather than metals may be the most important inorganic contaminates for irrigated agriculture in this region. Although metal concentrations in orchard soil showed an increasing trend of surface accumulation from irrigation, concentrations were comparable to those found in western US croplands (Holmgren et al., 1993) and well below global ranges in soil (Fergusson, 1990; Mengel and Kirkby, 1982). Metals in water were also below contamination ceilings for irrigation (Pratt and Suarez, 1990). An increase of metal inflow into the river is unlikely with the current more stringent regulation. Increased salt loads in irrigation water are a certainty. Local agriculture will increasing rely on urban effluents, too expensive to recover as potable water, for irrigation. Our original hypothesis suggested that solute distribution patterns would implicate the transport role of contaminants in irrigation waters. Instead, salts with the highest contamination potential showed limited-to-no spatial dependence.
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