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Environmental Geology 38 (3) September 1999 7 Q Springer-Verlag 199

Tentative nitrogen budget for pitlatrines – eastern BotswanaG. Jacks 7 F. Sefe 7 M. Carling 7 M. Hammar 7 P. Letsamao

Received: 27 February 1998 7 Accepted: 14 September 1998

G. Jacks (Y)Dept. of Civil and Environ Eng Royal Inst of Technology,S-100 44 Stockholm, Sweden,e-mail: Gunnjack@aom.KTH.SE

F. SefeDept. of Environments Sciences, University of Botswana,Gaborone, Botswana

M. Carling 7 M. HammarLuleå University of Technology, S-971 87 Luleå, Sweden

P. LetsamaoCentral District Council, Private Bag 001, Serowe, Botswana

Abstract A major problem with on-site sanitationis nitrate pollution of the groundwater. A tentativenitrogen budget is established for pit latrines ineastern Botswana. The ammonia volatilisation wasfound to be negligible while leaching varied largelyfrom about 1 to 50%. Leaching of nitrate was as-sessed by using chloride as tracer, assuming twosources of chloride, atmospheric deposition and theuse of common salt in food. The initial content ofnitrogen in excreta was assessed from nutritionaldata. The residual nitrogen in abandoned latrinesas found by analysis, was 15–20%. The remaindershould be denitrification which would then be inthe order of 30–70%. That denitrification is impor-tant is supported by an elevated N-isotope ratio ingroundwater and in deep-rooted non-N-fixing trees.The varying leaching rate provides a possibility ofchecking it by sealing the latrines. Since about 95%of the nitrogen in human excreta is present in theurine, an even more attractive solution would beurine-separating latrines with surface near percola-tion of the urine in the root zone of the vegetation,utilising it for crop growth. Since such latrines areused elsewhere in the world the problem is nottechnical but social acceptability.

Key words Nitrate pollution 7 Sanitation 7Groundwater 7 Botswana

Introduction

Pit latrines have greatly improved sanitation in develop-ing countries during the last few decades (Pickford 1991;Marks 1993). Especially children, at the age when theystart to move around, profit from the improved sanita-tion, avoiding parasitic and bacterial infestations (Cairn-cross 1987; Puddifoot 1995; Messou and others 1997).Giardia infections in infants during the first year of lifedecreased with increased use of latrines (Mahmud andothers 1995).There has been an extensive local development of differ-ent types of latrines such as the Ventilated Improved PitLatrine (VIP) in Botswana. Some latrines are designed tobe emptied regularly while most latrines are just pitswhich are abandoned once they are filled up. While thelatrines have improved the sanitation, they have, in someareas, led to deterioration of the groundwater quality bybacterial pollution and by increasing the nitrate content(Lewis and others 1980; Gbodi and Atawodi 1987; Reed1994, Nkotagu 1996).Well drilling for water supply was initiated in the 1960sin eastern Botswana. In the late 1970s, the nitrate contentincreased rapidly and the mean nitrate contents wereabove 50 mg/l in the mid 1980s. By moving the well fieldsout of the inhabited areas the nitrate content in the sup-plied water has again been lowered (Lagerstedt and oth-ers 1994). Nitrate in concentrations above 45 mg/l maycause methemoglobinemia (Fan and Steinberg 1996).Livestock losses are considered to be relatively commonin South Africa due to nitrate poisoning (Tredoux and duPlessis 1992). Reproductive toxicity has been suggestedfrom epidemiological investigations but no conclusiveevidence exists (Fan and Steinberg 1996). Conflicting evi-dence exists concerning the risk of contracting gastriccancer in connection with high nitrate contents in drink-ing water (Rademacher and others 1992; Morales-Suarez-Varela and others 1995; Yang and others 1997). High ni-trate levels may indicate an elevated risk for bacterialpollution (Craun and others 1981).

Geographical characteristics

This investigation was made in eastern Botswana in Mo-chudi and Ramotswa villages, which have 25 000 and

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200 Environmental Geology 38 (3) September 1999 7 Q Springer-Verlag

Fig. 1Ammonia contents in ventilation airfrom pit latrines measured during dayor night or for 24 h. M Mochudi, RRamotswa

Table 1Population and sanitation in Mochudi and Ramotswa villages

Village Number ofhouseholds

Populationdensitypersons/km2

Flushlatrines%

Pitlatrines%

Nosanitation%

Mochudi 5400 1250 8 69 21Ramotswa 3360 2900 10 79 11

20 000 thousand inhabitants respectively. In spite of theof size of these centres, they have the character of largevillages rather than urban agglomerations. The popula-tion density in eastern Botswana is about 180 persons/km2. The population density in Mochudi and Ramotswais 1250 and 2900 persons/km2 respectively. An estimatedgroundwater recharge (Gieske 1992) within Mochudi vil-lage would give a daily supply of about 45 l/person. Thewater supply varies from handpumps to standpipes andhouse connections. The sanitation is commonly in theform of pit latrines (Table 1).The climate is semi-arid with an annual precipitation ofabout 500 mm. The groundwater recharge is of the orderof 15–25 mm (Gieske 1992). The mean annual tempera-ture is 21 7C. The rocks are Precambrian granites andProterozoic sandstones, quartzites and shales. Thegroundwater in the area mostly occurs in fractures andjoints. The soils are ferric luvisols, ferric arenosols andregosols. The cover of loose overburden varies from nilto a few metres. The vegetation is a tree and shrub sa-vanna with Croton gratissimus, Acacia erubescens andTerminalia sericea as the most common species. Otherspecies are Boscia albitrunca and Boscia foetida. The cul-tivated area is less than 20% but the density of livestockis high, about one head of cattle and two goats/sheep perhectare.

Methods

Groundwater analyses were partly obtained from theDeptartment of Water Affairs and some were made at theDept. of Environmental Sciences at the University of Bot-swana in Gaborone. Chloride and nitrate were analysedby ion selective electrodes.Analyses of nitrogen isotopic ratios (15N/14N) were madeat Umeå University in Sweden for samples from vegeta-tion and at CSIR laboratories in Pretoria for nitrate ingroundwater. The plants sampled were deep-rooted non-N-fixing species, most of them Boscia foetida, while a fewwere Boscia albitrunca and Croton gratissimus. Thesetrees are supposed to extract water and nutrients fromthe groundwater which is generally found at 5–20-mdepth below ground level. Boscia albitrunca extends itsroots to 68-m depth in the Kalahari in Botswana (Cana-dell and others 1996).

Ammonia volatilisation was measured by placing diffu-sion tubes from the Swedish Environmental Research In-stitute (Ferm 1979) in the ventilation tubes on pit latrinesfor 12 h during the day, or in the night or for 24 h. Therecording is based on the principle of diffusion into anacidic filter in which the amount of ammonium caught isa measure of the ammonium content in the air and theexposure time. The ventilation rate was observed twice ateach site by introducing smoke at the exit of the venttube. Information on the number of users was obtainedfrom the owners.Tot-N, tot-C and tot-P analyses were made by the Kjel-dahl method, by Shimadzu TOC 5000 carbon analyzerand spectrophotometrically respectively.

Results

Four pit latrines were observed concerning the loss ofammonia via the ventilation. The number of users variedfrom 2 to 12. The content of ammonia was of the orderof 0.8 mg/m3 (Fig. 1) which is comparable to the concen-tration above a field where liquid animal dung has beenspread. There was no obvious relation to number of us-ers. The flow rates were small, in all cases less than 1 l/swhich made the losses comparatively small. The meanammonia volatilisation was about 60 mg N/24 h. The ni-trogen input was assessed to 11 g N/person per 24 h fromprotein a intake of 80 g/day (Maletnlema 1992). The am-monia volatilisation thus turned out to be a small frac-tion of the nitrogen budget when measured in the fourlatrines or of the order of 0.05–0.3% of the total input ofnitrogen into the latrines.The leaching rates from the pit latrines were assessed byusing chloride as a tracer. Chloride in the area is derivedfrom atmospheric deposition and from the use of cook-

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Fig. 2Nitrate plotted versus chloride for a number of wells inMochudi and Ramotswa. Filled symbols are latrines in welldrained soils from Ramotswa, while open symbols are latrines inimperfectly drained soils from Mochudi. The lines denotingdrainage loss take into account the fact that 10 mg/l of chlorideis derived from atmospheric deposition

ing salt by the population. The chloride deposition con-tributes 10 mg/l of chloride to the groundwater takinginto account a content of 0.4 mg/l in rainwater and a re-charge rate of about 20 mm from a rainfall of 500 mm(Gieske 1992). This is verified by a lowest recorded con-tent of chloride in groundwater of around 10 mg/l in ar-eas far away from habitations. The intake of common saltvaries in different communities in the world from 5 to25 g per person and day with most communities consum-ing 9–10 g per person per day (Stamler and others 1991).By assessing the use by comparable groups in neighbour-ing countries and consulting the Family Health Divisionof the Ministry of Health in Botswana we estimated theuse of common salt to 9 g/person/day. This correspondsto 5.5 g chloride per person per day. A number ofgroundwater analyses have been plotted in Fig. 2. In thediagram lines of leaching have been drawn for 100, 50, 10and 1% loss of nitrogen in the form of nitrate in relationto chloride, which is considered as a conservative tracer.When plotting the water analyses, 10 mg/l of chloridewere subtracted, considered to come from atmosphericdeposition. The leaching varies from slightly more than50% down to 1%. In another investigation with moresamples, a similar range was recorded (Lagerstedt andothers 1994). Four abandoned pit latrines were sampled.The contents were analysed for tot-N and tot-P. In as-sessing how much of the nitrogen is left in the residues,it was considered that phosphorus is conservative andthat it remains in the residues. This is a similar approachto that used by Ernst and Tolsma (1989) in assessing am-monia loss from soil around watering points in Botswa-na. The phosphorus intake can be quite accurately as-sessed from the protein intake, since the bulk of thephosphorus and nitrogen come from the same fooditems. Boaz and Smetana (1996) have given the regres-

Table 3N-isotope ratios in vegetation and groundwater expressed asdeviation from atmospheric N2

Samplingsite

Numberof samples

15N/14N ‰ sample

Mochudi 14 c15.7 (3.2)a Boscia foetida, Crotongratissimus

Ramotswa 11 c10.7 (1.6)a Boscia foetida, Bosciaalbitrunca

Ramotswa 1 c10.1 borehole 4336Ramotswa 1 c11.6 borehole 4357Ramotswa 1 c11.8 public tap

a mean value and standard deviation

Table 2Tot-N and Tot-P in abandoned pit latrine contents

Samplingsite

pH Nitrogen% of DM

Phosphorus% of DM

N/P ratio RemainingN %

Mochudi I 6.7 0.69 0.42 1.64/1 19Mochudi II 6.5 1.20 0.74 1.62/1 19Ramotswa I 0.67 0.45 1.49/1 17Ramotswa II 1.23 0.80 1.54/1 18

sion equation: phosphorus intake p 0.0141 (protein in-take) c 0.128 which gives the intake in grams if the pro-tein intake in g/day is introduced. With a protein intakeof 80 g/p/day the phosphorus intake will then be 1.26 g/p/day. Table 2 lists the results of the analysis of the contentof four abandoned pit latrines. The matter introducedinto the latrine was considered to contain N/P in the ra-tio 8.73/1 (11/1.26).The denitrification could not be assessed directly. Howev-er, three groundwater samples were collected in Ramot-swa and the N-isotope ratios with the air as a referencewere analysed. The ratios ranged from 10 to 12 indicatinga considerable positive shift, indicating a considerable de-nitrification (Heaton 1986). This range is similar to thatfound by Aravena and others (1993) in an investigationof a septic system. An indirect measure of the denitrifica-tion can be obtained from the vegetation samples (Table3). The 15N/14N ratio in natural vegetation, e.g. Terminal-ia sericea, in unpolluted sites in the region is around 0per mille (Heaton 1987) while a positive shift to 11–16per mille, indicating denitrification, is found here (Table3).

Discussion

The fate of nitrogen introduced into a pit latrine can bedivided into the following categories: ammonia volatilisa-tion, deep leaching of nitrate, denitrification and nitrogenin residues. The distribution as found in the results islisted in Table 4 and depicted in Fig. 3.

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Table 4Tentative nitrogen budget for pit latrines

Process Fraction Method of assessment

Ammonia volatilisation ~1% measurementNitrate leaching 1–50% use of chloride as tracerResidual nitrogen 17–19% analysisDenitrification 30–70% remainder

Fig. 3Major pathways for nitrogen in pit latrines. The percentagesstand for fractions of nitrogen introduced into the latrine

The low ammonia volatilisation depends on the neutralto slightly acidic pH in the latrine contents (Table 2) incombination with a low ventilation rate. As the pKa valueof ammonium is 9.3, only a tiny fraction will be in thevolatile ammonium form at a pH below 7. Nitrate leach-ing varies greatly and is often considerable which is ex-plained by the commonly light textured soils in easternBotswana. It is also based on the condition that there areonly two major sources of chloride, atmospheric deposi-tion and the use of common salt in cooking by the popu-lation and that the major source of nitrogen is from hu-man excreta. The absence of any sizeable industry ex-cludes this source of chlorides. Inorganic fertilisers arenot used and nitrogen in animal excreta does not seemto be extensively leached into the groundwater. Wellsaway from habitations are low in nitrate (Lagerstedt andothers 1994). Ammonia volatilisation removed a majorportion of the nitrogen from animal excreta in the neigh-bourhood of watering points according to Ernst andTolsma (1989). However, the recharge rate may vary con-siderably in this terrain (Gieske 1992), a condition thatwill affect individual data points. Nitrate leaching doesseem to depend on the texture of the soil although theevidence brought forward here is weak. The large rangerecorded in Fig. 2 as well as by Lagerstedt and others(1994) is understandable in view of the variable thicknessof loose overburden, its different drainage characteristicsand different designs of the pit latrines. A lining, usuallywith bricks, is provided only when the subsoil is not sta-ble enough to carry the load of the superstructure.The assessment of the fraction left in the residues de-pends on the validity of the assumption that phosphorusis conservative. Phosphorus certainly does not reach thegroundwater, but a fraction could be leached into themineral soil in the bottom of the latrine. If so, the frac-tion of residual nitrogen would be over-assessed. The as-sessment of the phosphorus intake by the formula pro-posed by Boaz and Smetana (1996) has been questionedby Agarwal and Agarwal (1997). Considerable deviationsfrom the formula could arise if the diet contains a sizablefraction of processed meat and cheese. However, this isunlikely to be the case here in these rather rural settings.The N/P ratio in Swedish sewage is 9.5/1 (Strid 1990)while the assessment used here for eastern Botswana is8.7/1.The degree of denitrification is a major question in con-nection with on-site sanitation (Fourie and van Ryneveld1995). Denitrification has been assessed as a remainderand not been measured per se. The positive values of the

15N/14N ratios in the three groundwater samples indicatethat the nitrate is a residue after a sizable denitrification.Nitrogen isotope fractionation occurs mainly in ammoniavolatilisation and denitrification (Aravena and others1993). In this case ammonia volatilisation is very small.Non-symbiotic, deep-rooted trees such as Boscia spp andCroton gratissimus show high 15N/14N ratios in theirleaves (Table 3). Canadell and others (1996) consider thatnitrate leached to the groundwater is recycled by deep-rooted trees. The ratios are significantly higher in Mo-chudi than in Ramotswa indicating more denitrification,possibly due to the less well drained soils in Mochudi ascompared to those in Ramotswa (Ministry of Agriculture1988). The well waters from Ramotswa have 15N/14N ra-tios in the same range as the trees from the same village(Table 3). The 15N/14N ratios ranging from 8.3 to 23 permille found in vegetation and groundwater reflects a de-gree of denitrification of 15–50% if compared with condi-tions recorded from Kalahari by Heaton (1984). This iscomparable with the relative fraction of 30–80% shown inTable 4 and Fig. 3.To avoid excessive leaching of nitrate, a number of meas-ures are possible. The ventilation tubes could be paintedblack to increase the ventilation rate in daytime. Increas-ing the pH by the addition of slaked lime would furtherincrease ammonia volatilisation. Sealing the bottom andthe walls of the pit could prevent leaching, and mightpromote denitrification. However, the most attractive al-ternative would be to separate the urine by a simple de-vice and infiltrate the urine into the nearby surface nearsoil utilising it for fruit trees or other deep-rooted crops.About 95% of the nitrogen and 65% of the phosphorus inhuman excreta are found in the urine (Drangert 1997).Urine-separating latrines have been developed, albeit foranother environment, in Kerala in India with a highgroundwater table (Calvert 1997). In the Kerala latrines,the urine is channeled to, and adsorbed in, a soil-filled

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trough beside the latrine. The soil can be exchanged andused as manure. The separation would, if accepted social-ly, be equally good in Botswana.

Acknowledgements We are thankful for support from the Swe-dish International Development Cooperation Authority for thefunding of two MSc theses which supplied part of the informa-tion for this article. We also acknowledge the co-operation withDr Siep Talma, CSIR laboratories in Pretoria, concerning N-iso-tope data.

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