Environmental Engineering Reference
In-Depth Information
Mean values between the time series in
Oct., Nov., and Feb. at each sampling
point denominated by the same letter,
and values within each time group
followed by the same number are not
significantly different at the 0.05 level.
Nitrate seasonal variation (Fig. 8.7c) at SW
1
shows an increase during November and a
sharp decrease during February. At SW
2
a pronounced increase was found in November,
but it was not statistically significant, most probably due to the high variance of the data
set. At SW
3
a significant decrease was found during the wet season. Spatial variations
indicate to a significant contribution from the farmland at both points.
Analyzing the possible pathways of nutrients transport, it should be considered that
mobility of phos-phorous (P) is chemically controlled and is dependent on the oxygen
content of the upper soil layer. Anaerobic conditions on the soil surface are contributing
to a significant release and leaching of dissolved P. Rydin (1996) reported that under
anaerobic conditions, 30% of the TP could be released from sludge. Irrigation with
wastewater and sludge of high organic content tends to create anaerobic conditions and
contribute to a P leaching (David & David 2000). Particulate P can be transported by the
drainage water or with eroded suspended particles. A well-defined correlation between
suspended solids and P concentrations in runoff has been found (Djodjic et.al. 2000,
Umemoto et al. 2001). Djodjic et al
.
(2000) reported that more than 50% of the TP in
drainage water was in particulate form, but a two times higher leaching was observed
from no-tillage plots in the form of dissolved P, and also that ponding conditions promote
P leaching. Grass plots tend to retain P better than other cultivated plots (David et al.
2000).
Due to chemical and biological transformations, organic nitrogen is transformed to
ammonia, and ammonia is nitrified to nitrite and nitrate consequently. Those processes
occur simultaneously at different depths of soil texture. Nitrate moves easily through soil,
because of its negative charge, and can be leached, while ammonia is retained in soil
texture due to its positive charge (Sukreeyapongse et al. 2001).
In a laboratory study of wastewater infiltration through a sand column (Reemtsma et
al. 2000), it was found that about 85% of ammonia was transformed to nitrate at a depth
of 1.2 m and the process of nitrification leads to the acidification of the upper layers of
the soil.
Considering the site conditions, it should be noted that approximately half of the farm
area drains into the Marimba River (SW
2
) and the rest into The Little Marimba River
(SW
3
). However, the former has a smaller flow rate and a dyke does not protect its banks,
leading to formation of wetlands. By visual inspection it was found that septic conditions
have been developed throughout the formation of wet areas, which are persisting during
the dry season. They provide an environment, conducive for the P and ammonia release
into the wetland water, which finds its way into the Little Marimba River, thus
contributing to significant increase of the nutrient at this point. Also, the banks of the
stream near and upstream SW
3
are not covered by grass, as it is the case near SW
2
, thus
