Tracking Sources of excess nitrate discharge in Lake Victoria, Kenya for improved Nitrogen use efficiency in the catchment B. Nyilitya, S. Mureithi, P. Boeckx 7th International Nitrogen Initiative 216 (INI), MCG, Melbourne, 4-8 December 216
Lake Victoria Nile basin- White Nile source Shared by 3 EAC states S.A = 68,8 km² C.A = 195, km² Kenya: S.A = 6%, C.A = 22% Uganda: S.A = 45%, C.A = 16% Tanzania: S.A = 49%, C.A = 44% Supports > 3 million people Nitrate Levels Increasing during the last 5 decades Lake: TALLING and TALLING (1966): 15 29 µglˉ1 GOPHEN et al. (21): 24.8 ± 21.8 µglˉ1 LUNG AYIA et al. (21): 66.6 ± 44. µglˉ1 GIKUMA-NJURU and HECKY (25): 48.2 ± 21.4 µglˉ1 L. SITOKI and C. EZEKIEL (unpublished): 98.7 ± 36.4 µglˉ1 Rivers: Raburu P and Okeyo J.B (unpublished): 4-6 mglˉ1
Eutrophication in Lake Victoria Attributed to sewage effluents & agricultural runoff (Lung ayia 21). Kisumu (Auji) stream Eutrophication effects in Lake Victoria water hyacinth, algal blooms Decrease in sechi depth (5 to 1 metre) hypoxia - decrease in fish population 1984(Ochumba, 199) Impairment of transportation, decrease in drinking water quality
Nitrate pollution management Policy and institutional framework is in place National and regional- WRMA, MW&I (Kenya), LVEMP, LVBC (EAC). Approach = water quality monitoring network (both lake and catchment stations). Assess nitrate concentrations, against 5 mgl -1, (WHO). But: Sources? How much each source?? Information Gap Amount of nitrate each source contributes to the observed nitrate in the river/lake
Study objective Apportion sources of nitrate discharged in major rivers (Nyando, Nzoia, Sondu) draining into Lake Victoria, Kenya, using isotopic (δ 15 N, δ 18 O - NO 3-, δ 11 B) techniques, hydrochemistry NO 3-, NO 2-, NH 4+ ) and mixsiar
12,79 km 2, 3,6 km 2 5,18 km 2, 18 m 3 s -1 42.2 m 3 s -1 115.3 m 3 s -1
High population density, Fast growing towns, industries, subsistence farming, medium-large scale farms - tea, maize, sugarcane, rice, horticultural crops and livestock farming
Study design Bimodal rainfall pattern, Spatial & temporal sampling Rainfall (mm) 25 2 15 1 Rainfall data Kisumu meteorological station (1967-1994) 25 24.5 24 23.5 23 22.5 Temparature ( c) 5 22 21.5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month 21
Results Nitrate (mglˉ1) show seasonal variation: July_216 (wet) Sep_216 (dry) Range.53 2.7.61 4.4 Mean.85 ±.45 1.1 ±.85 5 4.5 4 R. Nyando nitrate content (mglˉ¹) Nitrate discharge at river mouth July_216 = 21778 mgs -1 3.5 3 Sep_216 = 686 mgs -1 2.5 High N discharge during wet seasons- 2 1.5 July_16 Sep_16 Spatial variation: Mbogo Forest tributary Finlays flower farm- commercial flower farm under irrigation Masaita - densely populated (Londiani) area, intense mixed farms & livestock 1.5
Nitrate source identification Kisumu streams 45. NO₃- in precipitation 18O 35. 25. NO₃- fertilizer finlays denitrification borehole Nyando_July16 Nyando_Sep16 Auji_July16 Auji_Jan15 15. 5. NH₄+ soil nitrate fertilizer & rain manure & sewage Kisat_July16 Kisat_Jan15 L Vic_Jan15 Boreholes -5. -5.. 5. 1. 15. 2. 25. 15N Most samples - manure & sewage, soil nitrate Seasonal variation- enrichment in wet season, kisumu streams Denitrification- vector for borehole -.6 mgl -1
Nitrate source apportionment.4 Upstream catchment (wet season).4 Upstream catchment (dry season).3.3 Upstream catchment Natural forest and commercial tea growing zone Main source = soil N; 25-35%; highest in the forest site.2.1 NP M/S NFR NF SN mixsiar derived box plot of proportional contribution of 5 potential sources of nitrate discharged in river nyando.2.1 NP M/S NFR NF SN NO₃- F: low contribution in wet =low fertilizer application NO₃- F: major source in dry season = foliar fertilizers for tea are applied in dry season.
Nitrate Source apportionment- Midstream M/S dominates: 27-35% wet season 25 47% dry season Soil N: 2 28%, esp. wet season NITRATE SOURCE CONTRIBUTION (%).4.3.2.1 Midstream catchment (wet season).6.4.2 Midstream catchment (dry season) NP M/S NFR NF SN NP M/S NFR NF SN NP M/S NFR NF SN 21.6 2.7 15.8 23.3 19.9 2.1 2.3 15.2 17.3 16.7 13.1 18.4 26.5 26.8 28 32.1 14.8 15.7 22.8 11 M A S A I T A 2 M U S C O F I N L A Y S A I N A M O T U A R. Nyando nitrate content (mglˉ¹) 5 4 3 2 1 Mbogo1 Ainamotua Masaita1 Masaita2 Finlays Nyando_musco Nyando_Midwe Nyando Mouth July_16 Sep_16 Finlays
Nitrate Source apportionment- Downstream catchment Flat land (kano plains) M/S and SN had similar contribution =23% Turbid (> 5 ntu) water.25.2.15.1.5 Downstream (wet) NP M/S NFR NF SN.35.3.25.2.15.1.5 Downstream (dry) NP M/S NFR NF SN
Key observations & further studies 1. Several sources of nitrate discharge into river nyando: mixture a) manure & sewage main source in midstream catchments - high population density, mixed farms and sugar industry b) Significant nitrate fertilizer contribution from commercial farms Sugar, flower farms. To be Studied Nitrate fate and dynamics in the rivers Apportionment of manure and sewage sources (δ 11 B) Long term seasonal/ spatial trend of nitrate sources Groundwater nitrate source apportionment and susceptibility to surface water pollution
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