Indian Journal of Marine Sciences Vol. 30, June 2001, pp. 75-80 Sediment characteristics of Poonthura estuary (southwest coast of India) in relation to pollution K S Anila Kumary School of Applied Life Sciences, Mahatma Gandhi University Regional Centre, Pathanamthitta 689 645, Kerala, India and P K Abdul Azis & P Natarajan Department of Aquatic Biology & Fisheries, University of Kerala, Beach P.O., Thiruvananthapuram 695 007, Kerala, India Received 17 August 2000, revised 27 January 2001 Texture, redox potential and organic carbon of sediments in Poonthura estuary, Thiruvananthapuram, presently exposed to sewage pollution were evaluated over a period of one year. Textural study revealed the presence of highest percentage of finer particles at the sheltered upstream stations. Oxidation-reduction potential varied between −211 and +156. Strong negative potentials recorded from the most polluted stations while freshwater and oceanic stations always recorded positive values. Organic carbon widely varied between 2.4 to 83.3 mg/g, and showed enrichment always at the polluted zone. Organic carbon content was more than the prescribed limit for unpolluted estuaries at some stations and reflects the level of pollution in the estuary. Significant variation in organic carbon and redox potential during the monsoon season shows that better dilution can diminish the high organic load to a great extent. Sediments are indicators of the quality of overlying of untreated sewage from the disposing area leaches water and its study is a useful tool in the assessment into the estuary through the canal. The seriousness of of the status of environmental pollution. The level of this waste disposal problem has been realized lately organic carbon in sediments is reported to be a but no steps have been taken to study the effect of reliable index of nutrient regeneration and the productivity of a water body. Redox potential is a readily obtained description of organic matter within the sediment and of its oxidizing or reducing power. Oxidation-reduction activities at the sediment-water interface bear marked influence upon the estuarine chemistry and upon the type of organisms present in the sediment. Although the components and composition of sediments have received much attention, most of the work in India relates to the texture and organic contents of marine sediments1-5 and not much information is available on the estuarine material6-8. Poonthura estuary and the lower reaches of Karamana river (8° 25’-8° 30’ N latitude and 76° 55’- 77° 00’ E longitude) on the south-west coast of India (Fig. 1) near to Thiruvananthapuram city, is facing enormous load of untreated domestic wastes emanating from the fastly growing Thiruvananthapuram city over the last few years. The city sewage farm is located at Muttathara on the bank Fig. 1—Map of the Poonthura estuary, with study sites (St. I - of Parvathy-Puthanar canal which joins the estuary Karimbuvila, II - Moonnattumukku, III - Edayar, IV - Panathura, about 2.4 km down to Muttathara and a huge quantity V - Poonthura) 76 INDIAN J. MAR. SCI., VOL. 30, JUNE 2001 pollution except for the fragmentary work of Anila corer (5.5 cm internal diameter and 45 cm long). Kumary & Abdul Azis9. The present paper deals with These samples were transferred separately to clean, the sediment characteristics such as texture, redox dry polythene containers, homogenized well and were potential and organic carbon of the sediments in brought to the laboratory for analysis. Readings of Poonthura estuary and discusses the extent of redox potential were taken as soon as the samples pollution in the estuary. were brought to the laboratory using inert platinum electrode connected top pH-Eh meter (Pearson & Materials and Methods Stanley)10. A portion of each of these samples was Five stations, almost equidistant from one another dried at 60-70°C and pulverised to estimate the which representing different ecological conditions, organic carbon11. The oven-dried samples were were selected in the estuary for regular monitoring subjected to textural study by sieving and pipette (Fig. 1). Station I (Krimbuvila) representing a riverine method following Krumbein & Pettijohn12. station is comparatively free from sewage pollution and is exposed to freshwater influx. Station II Results and Discussion (Moonnattumukku) is located in the western segment The textural study revealed that, silt and clay were of the estuary where the sewage drains open directly the predominant fractions at stations I and II, whereas into the estuary and is heavily polluted. Station III the sand fraction was predominant at stations III, IV (Karimbuvila) represents the interior segment of the and V (Table 1). The percentage of fine fraction was estuary where the sewage is comparatively diluted. maximum (65%) at station II. Further, the sand Station IV (Panathura) lies in the estuary near the fraction shows an increasing trend and the fine estuary mouth into the Arabian Sea and station V is fraction shows decreasing trend from stations III to V. a typical beach station located outside the estuary In estuaries generally fine mud get deposited near the mouth. The depth of these stations varied head and other sheltered regions, while along the from 0.5 to 3.5 m. main channel and at the mouth coarser sediments are Sediment samples (triplicate) were collected from deposited. Weak tidal currents in the upstream and the all these stations at monthly intervals for a period of heavy land drainage could be the reason for the one year (February 1992-January 1993) using a metal dominance of silt at station I. However, the mixing up Table 1—Monthly variation in percentage eight of sediments grain size composition at different stations St no. Sediment Feb Mar Apr May June July Aug Sept Oct Nov Dec Jan type I Sand 29.25 32.50 41.21 27.85 51.55 42.35 22.30 31.65 28.73 36.78 52.25 46.85 Silt 65.46 61.35 56.85 64.75 39.28 46.83 76.52 62.25 55.25 60.95 44.72 45.85 Clay 5.29 6.15 1.94 7.40 9.17 10.82 1.18 6.10 16.02 2.27 3.03 7.30 II Sand 22.10 38.65 26.38 32.40 41.25 36.12 38.15 37.85 35.28 39.85 38.13 33.58 Silt 70.67 42.88 38.31 29.55 52.35 58.75 55.29 40.55 52.25 50.95 48.64 50.38 Clay 7.23 18.47 35.31 38.05 6.40 5.13 6.56 21.60 12.47 9.20 13.23 16.04 III Sand 48.50 56.78 42.15 57.54 49.88 79.37 58.45 60.15 52.83 48.15 59.35 44.18 Silt 41.95 40.65 34.83 31.87 47.74 18.55 38.15 27.43 44.54 44.94 39.18 46.30 Clay 9.55 2.57 23.02 10.59 2.38 2.08 3.40 12.42 2.63 6.91 1.47 9.52 IV Sand 63.35 55.95 59.29 74.75 93.25 100 81.58 80.15 62.75 76.52 53.43 41.75 Silt 30.63 40.35 34.24 24.25 6.75 0.00 10.20 17.45 36.85 23.20 40.58 53.25 Clay 6.02 3.70 6.47 1.00 0.00 0.00 8.22 2.40 0.40 0.28 5.99 5.00 V Sand 72.00 84.15 59.90 95.00 100 100 95.00 90.80 100 96.00 100 90.50 Silt 10.80 12.35 28.50 5.00 0.00 0.00 3.50 7.55 0.00 2.80 0.00 2.50 Clay 17.20 3.50 11.60 0.00 0.00 0.00 1.50 1.65 0.00 1.20 0.00 7.00 ANILA KUMARY et al.: SEDIMENT CHARACTERISTICS OF POONTHURA ESTUARY 77 of great amount of sewage and the low energy Redox potential on one hand is related with the condition allowing the settling of fine particles distribution of oxygen and on the other hand, with the favored the enrichment of silt at station II. cycles of most of the important elements occurring in Oxidation-reduction potential (Eh) of sediments is water. Hence the redox state in a given place is a convenient index as to whether or not the sediment influenced to a great extent by hydrodynamical and is reducing in reaction. A positive Eh reading result sedimentological conditions13. Riverine zone was from a state tending towards oxidation, and a negative characterised by strong water flow and high Eh indicates a system causing reduction. The concentration of dissolved oxygen in the overlying predominance of positive values at stations I, IV and water, which could lead to the strongly oxygenated V, and negative values at stations II and III (Fig. 2), conditions. The oxygenated conditions at the marine suggests that the marine and riverine sediments are zone can be attributed to the highly porous medium of less reduced than interior estuarine sediments. The coarse sediment, which made a high oxygen diffusion monthly values varied between −211 (station II) and rate possible. The rearrangement of sediment by +156 mv (station V). Table 2 also shows that most strong hydrodynamic action could also be reduced sediments were present at station II and most contributing to healthier sediment at the marine oxidised conditions at station V. Variations between zone14. stations were significant at 1% level (ANOVA: A distinct seasonal influence was discernible on the DF = 4, F = 26.6201). variation in the oxidation-reduction potential. All the Fig.