SCIENTIFIC CORRESPONDENCE

Egypt, situated between the Sahara and the Eastern deserts, is hotter and drier Salinity threat to Upper Egypt than in the Colorado Basin. SIR-Salinization of land in arid regions The only remaining process that could It therefore appears that evapotrans­ has led to a decrease in the fertility of soils account for the high salinity of the ground­ piration is an important process signifi­ in many areas of the world. In Egypt, water is evapotranspiration. If that were cantly contributing to the salinity of the salinization poses a threat to agricultural the only factor responsible for the groundwater in the Karnak area. Since productivity as well as to the survival of observed groundwater salinity, the impli­ 96% of the groundwater, which flows many antiquities, mostly in irrigated land. cation would be that 89±3% of the water from the irrigated fields into the unirrig­ Leaching of accumulated salts ated land where the Karnak Temple Com­ by annual inundations previ­ Na+ +K+ concentrations and concentration ratios plex is located, evaporates under the ously prevented land saliniza- (1969-1987) temples', practically all of the salts origin­ tion in the Nile Valley, but that (meq/L± s.d.) ally present in the irrigation waters are ceased when the Aswan High being deposited at the ground surface and, 1 2 Groundwater Nile water Concentration Dam was completed in 1965 ' , ratio more devastatingly, in the foundations of By 1977, new salt deposits had 1969-70 9 ±2* 1.1 ±0.2 8±2 the monuments at Karnak. Because been observed at the foun- 1977 10.6±0.2 1.52±0.03 9±2 irrigated fields are located along the Nile dations of ancient monuments, 1987 10.2±0.7 0.99±0.03 9±2 River over a large part of Egypt, we but the reasons for the increase Average 10.0±0.7 1.2 ±0.3 9±1 expect that the Nile Valley south of of groundwater salinity have 3 4 Fayum will be affected much as we have been far from clear. *Average of nine wells collected throughout Karnak ' . found the Karnak region to be. We selected the Karnak/ G. BURNS Luxor region for our studies because it is now used for irrigation evapotranspirates T.C. BILLARD representative of archeological sites in at the ground surface of the cultivated K.M. MATSUI Upper Egypt. It includes the Karnak fields. This result is in itself remarkable. Department of Chemistry, Temple, one of Egypt's most important In the Colorado River Basin, for example, University of Toronto, archaeological monuments, located approximately only 75% of the water Toronto, between irrigated fields and the Nile applied to irrigated fields is lost to evapo­ Ontario, 1 2 River ' • transpiration'. The climate of Upper Canada MSS 1A1 Samples of groundwaters from the headquarters of the Canadian Archaeo­ logical Expedition immediately east of Leaky answer to greenhouse gas? Karnak and of Nile water near Karnak SIR-The British government is their recent substantial pipe-replacement were collected in 1977 and 1987, were encouragingly positive towards action to works, British Gas has cut this from values analysed to determine Na+ and K+ con­ reduce global warming from an increasing that were as high as 15-20% . The present centrations using atomic absorption spec­ 'greenhouse' effect, using in particular 'unaccounted for' rate is 3-4% in Wales, troscopy and were then compared with arguments endorsed by the House of little different from the UK average (J. 1 data assembled earlier·•. It was deter­ Commons Energy Committee , that Burrows, personal communication). mined that the concentrations of these favour electricity generation from gas British Gas told the Energy Committee1 ions in the Nile fluctuate within merely rather than coal. The case is based on the that methane leakages from the gas grid ±25% over a period of eighteen years (see relative carbon coefficient of various amount to 400,000 tonnes per year, nearly table). We have taken these ions as a fuels, essentially a measure of CO, emis­ 1% of throughput, but this is a politically proxy for salinity for that reason, because sion per unit of energy, and which are 0. 75 motivated lower limit. Household gas they are the most abundant ions in for coal, 0.62 for oil and 0.43 for gas'. But meters are accurate to ±2% so I will groundwater and because they are does that argument hold up? assume 2-5% as a realistic range for leak­ expected to be relatively unreactive with Methane makes up 75% of natural gas, age from the low-pressure delivery net. the soil. As an index of groundwater salin­ has a stronger greenhouse effect than CO, Leakage rates for sea-based production ity, we took the ratio of the Na++K+ and its concentration in the atmosphere are unavailable, but I estimate it to be concentration in the groundwater sample is increasing faster. Because methane 1-5%, based on information from the Pip­ to the average concentration of these ions (CH4) blocks radiation of wavelengths at er Alpha disaster inquiry. This estimate of in the Nile River at Karnak. If these high which the atmosphere is relatively trans­ 3-10% total losses is similar to the 3-6% concentrations of Na++K+ in 1969 were parent, its 'greenhouse' strength per derived for land-produced natural gas in due to the previous inundation regime, molecule is about 25 times stronger than the United States\ where there are legal they would have been leached out that of CO, (ref. 1). Releases of methane restrictions and accounting for leakage. between 1969 and 1987. That there has associated with coal mining and gas Hence the relative greenhouse effects, been no such effect is not surprising: production may be significant'·' relative to correcting for the 3-10% leakage and irrigation in Egypt has been stable for = the biological production. The present expressed per carbon atom, are G8.,:Gro., millennia. increase of 1% per year (several times that [0.43 + 110.75 (0.03 to 0.1) x 25] : 0.75 = Alternatively, high groundwater salin­ for CO,) may reflect decreased OH 0.57 + (0.75 to 2.5) 11, where 11 is the ratio ity could have arisen by the diffusion of density due to pollutants such as CO, but of atmospheric residence times for CH, salts in soils in the nearby desert, in part reflects the increasing release rates. and CO,. Evidently the value of 11 is signi­ previously unaffected by irrigation, into The production and delivery of natural ficant. The effective residence time for agricultural areas. If such diffusion had gas necessarily involves leakage - at the CO, is uncertain, whereas that for the light been significant, a time-dependent con­ sea well-heads and high-pressure pumps CH, is the stratospheric lifetime of 6.5 centration gradient would have been (to 1,500 p.s.i.), at the shore hand-over years (ref. 4) augmented by the upward generated, but none has been observed. and booster station, and in the local dis­ diffusion time to a total of about 10 years. 1. Billard, T.C., & Burns G. Nature285, 654- 655 (1980). tribution net and beyond the meter. Gas CO, interchanges with oceans and bio­ 2. Billard, T.C. thesis, UniversltyofToronto (1985). leakage in the local net can be judged on mass, a substantial fraction being 'fixed' as 3. Traunecker, C. K~mi20, 213- 228 (1970). the basis of the metered difference the annual atmospheric loading is only 4. Traunecker, C. K~mi21, 177- 196 (1971). 5. Pillsbury, A. F. Scient. Am. 55, 55-63 (1981). attributed as 'unaccounted for'. Through 50% of the global fossil-fuel burn. The NATURE · VOL 344 · 1 MARCH 1990 25