The condition of the water quality in the Tha Chin River (TRC) Basin is known, but the knowledge of the sediments (especially regarding nitrogen concentration) remains low. Sediments could play an important part in receiving nutrient pollution (as nutrient sinks) in the TRC. This study investigates into the relation between canal water quality, land-use and sediment characteristics in Thasarn Bang Pla canal, which is representative canal in the Tha Chin River Basin, with a main focus on nitrogen contributions of sediment in the canal. The dominant agricultural land use types in Tha Chin river basin are raised bed (fruit & vegetable cultivation), rice field, cage fish, and shrimp farm. Canal water and sediment characteristics at 15 sampling points in the Thasarn Bang Pla canal were measured and interpreted, to demonstrate the influence of discharge from different land use types on the sediment nitrogen trend in the canal. The study period covered 6 months (October, 2006 to March, 2007).
To understand the nitrogen characteristics in the canal sediments, the following parameters were considered: nitrogen concentrations, pH, dissolved oxygen and temperature in the water and sediment, as well as total solids, settled solids, organic matter and grain-size compositions of the sediment. The results of water quality observation indicated that Thasarn Bang Pla canal achieved the Thai water quality standards for “moderately clean fresh surface water resources used for consumption, requiring special water treatment process before using”. The major constituents of sediment in Thasarn Bang Pla canal was sand, with low organic matter (1.73-16.91%). The nitrogen in the sediment was mainly present as the combined form of organic nitrogen with some amount of ammonia and nitrate. The average total nitrogen content in the sediment was 1.597 g/kg.
The nitrite and nitrate concentrations presented in the sediments affected by rice field, raised bed and shrimp farms. Nitrite was range from 0.0008 to 0.0074 g/kg and nitrate ranged from 0.001 to 0.027 g/kg. This may be due to fertilizer application and feed input from the respective land use. Ammonia concentration in the canal sediments appears to be similarity affected by all land use types (range 0.006-0.138 g/kg). For cage fish, ammonia is mainly due the waste from fish. Ammonia concentration in the sediment affected by raised bed, rice field and shrimp farm are assumed to be the result of nitrificationdenitrification processes related with fertilizer application.