Assessment of Chemical Retention of Riparian Area (Thaulla) in a Small Tank: A Study in Kankanam Tank, Weeraketiya, Sri Lanka

Abstract

The agriculture sector in Sri Lanka utilizes a considerable amount of land cover (27,400 km2
),
allowing a significant contribution to the national economy. Most of these agricultural fields are
cultivated via irrigation schemes, especially in the Dry Zone of Sri Lanka, which receives limited
rainfall throughout the year. In ancient days, communities in these areas used their techniques to
develop tanks as a strategy to store and conserve the rainwater. The riparian area or “Thaulla” located
in the upper peripheral region of the tank, is one of the basic and vital components that help filter
nutrient pollutants and sediments in the water inflow of the tank. This study aimed to assess the
chemical retention of Thaulla in a small tank viz. Kankanam tank, located in Hambantota District in
Sri Lanka. The extent in the Thaulla area of the selected tank is about 2,185.3 m2
. Surface soil (0-30
cm) samples from eleven locations within the Thaulla area and water samples from three tank
locations were collected randomly. As the control, typical soil samples near the sampling site were
collected. These samples were tested using standard methods for nine parameters: pH, EC, NO3
-
-N,
NH4
+
-N, Total N, P, Na, K, and Ca. ArcMap Software prepared a spatial distribution map for each
parameter to identify their spatial variations in the studied area. Results revealed that concentrations
of nutrients in Thaulla area were high compared to the typical soil and water in the tank indicating
that it was highly contributed to retaining chemicals such as P (29.88 ppm), Ca (447.55 ppm), Na
(5.14 ppm), and K (4.17 ppm). According to the spatial variation, it was noted that various external
sources around the tank influenced the nutrient retention in the Thaulla area. The nutrient retention
was high in the areas, closer to the landfills, paddy fields, and leachate canals. Further, the
functionality of the Thaulla area as a constructed wetland could be identified. The tank water quality
was satisfied with the tested parameters and in accordance with the irrigation water quality guidelines.
Therefore, the Thaulla/riparian area is an important component of a tank allowing substantial
chemical retention to control the pollution of water in the tank. Further studies need to be conducted
to understand the seasonal variation of the chemical retention in Thaulla area.

Keywords: Constructed wetland, Riparian area, Small tank, Soil parameters, Water quality parameters