Caracterização físico-química e sanitária das águas superficiais usadas na irrigação de hortaliças (alfaces, Lactuca sativa L.) e dos solos irrigados nos municípios de Lagoa Seca e Sapé (PB).

Abstract

This research aimed the evaluation of irrigation water and soil used in lettuce [Lactuca sativa) culture including the crop itself in five vegetable-gardens located in two municipalities near Campina Grande, Paraiba State, Northeast Brazil. Experimental work was carried out in two vegetable-gardens in Sape municipality (7°06!15"S; &mTN) and three in Lagoa Seca (7°10'15"S; 35°51'13*W). Samples of irrigation water, soil and lettuce were done twice a month during rainy season (february to july/96) and dry season (september/96tojanuary/97). Water samples were analysed for temperature, pH, BOD5, total solids and fractions, alkalinity, nitrogen, phosphorus and calcium, chlorophyll "a", faecal coliforms and faecal streptococci. Carbon,(organic matte^and ion concentrations (calcium, magnesium, sodium)^re^de1emin¥d3n soil samples. Faecal contamination measured as faecal coliforms, faecal streptococci and £ coli were evaluated in soils and lettuces from the five vegetable-gardens and from local markets. Water pH was uniform near to neutral not affecting crop growth. High values of 8,3 were associated to algae photosynthetic activity (positive and significant between pH and chlorophyll: r = 0,4854; a = 0,05) and soil chemical composition, rich in carbonates and bicarbonates. This feature alsoraised valuestof alkalinity and hardness (positive and significant correlation between hardness and calcium: r = 0,9162, a = 0,01; and hardness and bicarbonates: r=0,9568, a = 0,01). Electrical conductivity was high, particularly during dry season with values between 704 and 1394 !imhos/cm due to salt concentration as a result of intense water evaporation (positive and significant correlation between electrical conductivity and susje|dMtoJfco|dsir = 0,5498, a = 0,01; and with fixed total solids: r =0,5375, a = 0,01). Domestic sewage contribution aiso increased electrical conductivity in two vegetable-gardens in Lagoa Seca (positive and significant correlation between electrical conductivity and BOD5: r = 0,3631, a = 0,05 rainy season; r = 0,5098, a = 0,05 dry season). Dissolved oxygen fluctuations (saturations values varying from 14% - rainy season to 115% - dry season) were related to increasing photosynthetic activities due to raise of insolation hours. Sharp fluctuations of BOD5 from 1 to 28 mg/L were observed, associated with organic material drifting and increasing with algal biomass. Concentrations of calcium, magnesium, sodium and potassium were variable in all water sources and seasons. Higher concentrations at dry season were appropriate to irrigation. Higher values of potassium were due to domestic sewage contributions(positive and significant correlation between potassium and faecal coliforms: r = 0,8800, a = 0,05; and with potassium and ammonia: r = 0,9831, a = 0,01). Sodium values were related with water evaporation. Water source Riverside classification was C3S1 not being suitable to soil with pour drainage. Accordingly to the nitrogen and phosphoruj> c o n c ^ n t ^ were classified as polieutrophic and hipereutrophic. Soils fertilised with cattle manure had basic pH (7,7 to 8,3) compared to the slightly acid non-fertJiised soils (5,7 to 6,5). Fertiliser added to soil increased 1,2 - 5,7 times the calcium, magnesium and potassium concentration when compared to non fertilised soils. Sodium concentration increased 22 times and available phosphorus was 41 times higher in fertilises soils. Nitrogen concentration also increased in manure fertilised soils. The C:N ratio varied from 0,14 to 9,66 in non fertilised soils and from 10,1 to 32,5 in fertilises soils. Ratio of 1:10 is considered appropriate (Emater-PB, 1979). Microbiological analysis in water, soil and crop showed significant fluctuations of all indicators. Only 12% of water samples presented < 1000 FC/100mL, recommend by WHO (1989) for irrestricted irrigation. Higher contamination occured in a vegetable-garden irrigated with water from a stream polluted domestic sewage. During dry season water sources presented the higher values of faecal coliforms and this contamination was associated with its intensive use, and due to entrance of contaminated organic material (positive and significant correlation between faecal coliforms e BOD5: r = 0,4732, a = 0,05; and between faecal coliforms and suspended total solids: r = 0,4295, a = 0,05; and between faecal coliforms and total phosphorus: r = 0,4992, a = 0,05). Lettuces from experimental vegetable-gardens showed high values of faecal coliforms (103 - 106 MPN/g) and soils fertilised with cattle manure presented higher numbers of indicator bacteria ( 105 — 106 CFU/g) suggesting the cattle manure contribution to soils and lettuce faecal contamination. Higher fluctuations were observed during dry season. Lettuces sold in local markets presented mean faecai contamination of 104 - 105 MPN/g. Showing high faecal contamination as those from the vegetable-gardens. All samples where faecal coliforms were present also contained £ coli indicating the faecal origin of contamination. Therefore these irrigation waters used in crops to be eaten raw did not fulfil CONAMA (20/86) standards due to high faecal contamination. The lettuces irrigated with these waters and those solds in local markets were also contaminated and not reaching the DINAL (1987) standards. Although the addition of cattle manure increased soil fertility, raising concentrations of phosphorus, nitrogen and organic matter, both fertilisation and contaminated water were responsable to high soil contamination.