4.2 Agricultural Return Flow Salinity as a Function of
Irrigation Loading and Soil Type
Agricultural return flow salinity was modelled using 1-D reactive
transport with an evaporative loading applied to the top 3 cells as
described earlier. Arabian Gulf seawater was selected as the irrigation
feed water and model layers were populated with mineral composition
using site-specific information from the Abu Dhabi Soil Survey. AD134
was a strongly gypsic soil (Table 3(a) ) with high gypsum
occurrence in all cells or layers, whereas AD146 (Table 3(b) )
was a carbonate-rich soil with almost no gypsum presence. Finally, soil
type AD157 (Table 3(c) ) was a mildly gypsic soil with high
gypsum content only in cells 3 and 4 from the ground surface. Three
different annual irrigation water loadings, 2.5 m, 3m, and 4 m, were
applied to each soil column model corresponding to leachate fractions of
0.25, 0.50, and 1.0, respectively.
The results from the reactive transport modelling for the three soil
types after one calendar year of continuous flow are presented inFigure 5 . Increased irrigation loading was found to generally
correlate with a decrease in return water salinity. This result appears
to corroborate the “salt penalty” effect described by
Edward P. Glenn, J. Jed Brown, and Eduardo
Blumwald (1999), which essentially attributes this effect to a
combination of lower dissolution of minerals from the soil at faster
flowrates and an averaging of the mineral dissolution over the larger
volumetric flow. The highest return water salinity (i.e., 56,500 ppm TDS
and higher) was observed for the high gypsum containing AD134 soil type
using the B-dot activity coefficient model. The discrepancy in salinity
prediction using the two different models was most pronounced for this
soil type, with PHRQPITZ predicting a salinity in the range of only
53,500 - 54,000 ppm TDS. Meanwhile soil types AD146 and AD157, the
carbonate-rich and gypsum-deficient soils, displayed a close match
between the two different activity coefficient models employed. These
results suggest that unlike the PHRQPITZ model, the B-dot activity
coefficient model appears to estimate a much higher solubility for
gypsum at higher ionic strengths. This result would require field
verification using soil core analysis during initial pilot studies.