BACKGROUND The Instituto Tecnológico de Oaxaca (ITO) and San Diego State University, San Diego, California (SDSU), have signed a cooperative agreement in March of 2000. This agreement stipulates the "exchange of students and professors, and the joint development of research projects that have a scientific or technological purpose." As part of this agreement, Dr. Victor Miguel Ponce, professor of Civil and Environmental Engineering at SDSU, has presented a research proposal to the program Research, Scholarship, and Creative Activity (RSCA) of SDSU, which was approved in December 2000. The proposal is entitled "Design of constructed wetland for the city of Tlaxiaco, Oaxaca, Mexico." The activities that form part of this study are to be accomplished during the period from January to June 2001. OBJECTIVE The objective of the study is to contribute to the cleanup of the Tlaxiaco River, which crosses of NNE to SSW the city of Tlaxiaco, in the high mixteca region of Oaxaca, following approximately the alignment of Mexico road No. 125, and crossing it in two places. Presently, the river and its tributaries are polluted by sewage waters that flow directly to the river courses. There is also the presence of trash, of both organic and inorganic origin, which is dumped by some local people, apparently without any control, into the watercourses. This situation has developed over the past 30 years, when the practice of sewage disposal changed from retention in latrines to drainage into the local watercourses. Population growth, both urban and rural, has gradually contributed to the current situation of the Tlaxiaco river, which is, nowadays, practically a sewage collector and a trash dump in some areas. The cleanup of the Tlaxiaco river will bring forth benefits to the community, including improvements in public health, water quality, recreation and wildlife habitat. Therefore, this study encompasses the actions necessary to implement the cleanup, including structural and nonstructural practices. This document constitutes the report of the visit of January 2001. DATE OF THE VISIT Dr. Victor Miguel Ponce visited the city of Tlaxiaco to learn first-hand the extent of the pollution of the Tlaxiaco river. The visit took place from Thursday, January 18, to Tuesday, January 23, 2001. On January 18, Dr. Ponce met with the Municipal President, Prof. Manual Gomez Martinez, and together they visited the river. That same afternoon, and during the following four days, Dr. Ponce visited the river and its tributaries, accompanied by Dr. Jose Perez, Councilman of Ecology. On Tuesday, January 23, Dr. Ponce presented a report to the Municipal Presidency on the scope of the work accomplished that week and on the future perspective. FIELD WORK ACCOMPLISHED During his visit to Tlaxiaco, Dr. Ponce inspected the river and its tributaries, to visually examine the extent of pollution and to learn in more detail the drainage system of the city. The city of Tlaxiaco consists of the urban center and six surrounding districts: San Diego to the north, San Nicolas to the northwest, San Sebastian to the west, San Miguel to the northeast, San Bartolome to the east, and San Pedro to the south. The Tlaxiaco river was inspected from the Albergue Reto (north of the city) to the property of Mr. Raul Jimemez (approximately 5 km south of Albergue Reto). In the vicinity of the urban center, the river has four tributaries on its right margin (west), from north to south: San Diego 1, San Diego 2, San Nicolas 1, and San Sebastian. Aditionally, San Nicolas 2 is a tributary of San Sebastian. The tributaries of the left margin (east) are: San Bartolome and San Pedro. The San Diego 1 and San Diego 2 tributaries drain rural areas north of the city; the San Nicolas 1 tributary drains rural and urban areas north of the city; the San Nicolas 2 tributary drains rural and urban areas west of the city; the San Sebastin tributary drains rural and urban areas southwest of the city; and the San Bartolome and San Pedro tributaries drain rural areas to the east and south of the city, respectively. In addition to the natural drainages, the system features two collector drains, which drain primarily the urban center of the city of Tlaxiaco. Collector A (Center-Colonia Benito Juarez) deposits its load adjacent to the property of Mr. Raul Jimenez, south of the city. Collector B (Center) deposits its load close to the center of town. Both collectors transport sewage waters, with high content of organic matter. ACTUAL SITUATION The city of Tlaxiaco deposits its sewage waters directly to the Tlaxiaco river, without any treatment. This produces the pollution of the river, which is incapable of cleaning (oxidating) the effluents (organic matter and other contaminants) within the available space and time. The actual conditions are summarized below. In Tlaxiaco there are distinctly rural and urban areas; the rural areas are principally in San Diego to the north; San Miguel and San Bartolome to the east, and San Pedro to the south; the urban areas are principally in San Nicolas to the northwest and San Sebastian to the southwest. The center of Tlaxiaco is mostly urban, and generally paved. The current sanitary instalations are of two types: (1) without drainage, by the use of latrines, and (2) with drainage to the natural or artificial system. The current latrines are of three types: Excavated pit, with standard waterclose, of wet operation. This pit is cleaned periodically, or sealed and replaced after a few years, depending on the use. Excavated pit, with or without place to sit, of dry operation. This pit is cleaned periodically, or sealed and replaced after a few years, depending on the use. Ecological latrine, constructed above grade, of two compartments, and separation of liquid and solid matter by means of the ecological chair, of two conduits. The compartments are used alternately, every two years or more, depending on the use, and the dry solid matter is eventually recycled in the field. The latrine users do not pollute the river or its tributaries directly. However, the excavated pits can indirectly pollute the watercourses, depending on their horizontal proximity to the watercourses, their vertical proximity to the water table, and of the type of construction (lined with concrete or other semi-permeable material, or unlined). The ecological latrine does not contaminate the watercourses and, properly operated, it can be used indefinitely. The users which have sanitary instalations connected to the natural or artificial drainage are directly responsible of the pollution of the river and its tributaries. PLANNING STRATEGY Analysis of the current sanitary conditions in Tlaxiaco enables the development of the following strategy: The inhabitants of rural areas should construct new latrines or improve the existing ones, with aim to reduce the quantity of sewage waters. The ecological latrine is preferable because it eliminates the possibility of indirect contamination and makes possible the recycling of the organic matter. The rural areas are mostly in San Diego, San Miguel, San Bartolome and San Pedro. The more latrines, the less will be the quantity of sewage waters, and the less the cost of treatment. Preliminary appraisal, based on the proportion of rural to urban areas in Tlaxiaco, indicates that at least 50% of the users should use latrines. In the urban areas, the properties are of lesser extent, and the drainage of sewage waters is already established by custom. Therefore, this load should be treated in one or more strategic places, prior to its delivery to the river or tributaries. The urban areas are estimated to be located mainly in San Nicolas and San Sebastian. The users connected to the drainage system should pay their share of the cost of treatment. On the other hand, the users not connected to the drainage system should receive technical and economic help to build new latrines or improve existing ones. In this way drainage is discouraged and latrine use is encouraged. Currently there exists two collectors, 1 and 2, which drain the center of the city and deposit their loal directly into the Tlaxiaco river. The possibility for treatment of these loads should be studied, prior to their delivery to the river. This could be accomplished with a system in series of oxidation pond and constructed wetland, for each collector, or for the combined load. The possibility of collecting the sewage waters which drain San Nicolas 1, San Nicolas 2, and San Sebastian should be studied, with the objective of treating these waters separately from the present collector system, or in combination. These tributaries have several local collectors, which would have to be combined and conveyed for treatment. The Municipal Presidency should develop and execute a program of environmental education, with aim to eliminate the practice of using the river and its tributaries as trash dumps. This is a difficult problem which requires special attention. PLAN OF WORK The plan of work has been divided into the following phases: A. Field work: Priority 1 Determination of existing and future sewage loads in collectors 1 and 2. The following data is required: Number of families which are currently draining to each collector. Number of families which would be draining to each collector in the year 2025. Estimation of current industrial sewage loads and projections to the year 2025. Compilation of available climatological data for the city of Tlaxiaco, including precipitation, temperature, relative humidity, evapotranspiration, and insolation. Topographic maps at scale 1:2,000 of the areas adjacent ot the river in the vicinity of the current location of the collectors 1 and 2. It is likely that treatment will be accomplished in these locations. Sampling and testing for water quality in the Tlaxiaco river, from the Albergue Reto to the property of Mr. Raul Jimenez. Ten (10) samples are estimated to provide a good baseline data. These samples are to be analyzed at the laboratories of the Instituto Tecnologico de Oaxaca. The following laboratory tests are required: Biochemical oxygen demand (BOD). Chemical oxygen demand (COD). Total suspended solids (TSS). Total dissolved solids (TDS). Oils and fats. Estimation of construction costs in the city of Tlaxiaco, prior to the preparation of budgets. B. Field work: Priority 2 Determination of existing and future sewage loads in the collectors that drain San Nicolas 1, San Nicolas 2, and San Sebastian tributaries. In this case, it is necessary to separate the rural areas, which do not drain, from the urban areas, which drain. Alternatively, it is necessary to separate the families (or users) who drain from those who do not drain. Field and topographic studies with aim to determine how and where to collect the sewage waters which drain San Nicolas 1, San Nicolas 2, and San Sebastian tributaries. Sampling of the water quality in local wells. Ten (10) sampling locations should be selected in strategic points around the rural portions of the city. These samples are to be analyzed at the laboratories of the Instituto Tecnologico de Oaxaca. C. Office studies: Literature review on the following topics: Oxidation ponds Constructed wetlands Ecological latrines Analysis of the existing topographic, hydrological, and climatological data. Design of the oxidation pond and constructed wetland on site(s) to be determined. Preparation of drawings. Preparation of costs and budget. CONCLUSIONS The following conclusions have been reached in this study: It is possible to clean the Tlaxiaco River, but this will require the construction of certain treatment structures, which have advantages and disadvantages. The size and cost of the treatment structures will be reduced by the establishment of a program to support the rural inhabitants, so that they may construct new latrines or improve existing ones. This practice avoids drainage to the municipal system and reduce the possibility of system failure due to overuse. For the treatment of the sewage waters, a system in series of oxidation pond and constructed wetland is proposed. This system is of low initial cost, low O&M cost, sustainable, and in line with local economic realities. The system will occupy areas adjoining the river, and the effluent will be of secondary quality, i.e., at least 90% reduction in BOD). The treated water can be deposited directly in the river, or be conducted through appropriate canals to the existing irrigation systems. The Municipal Presidency should develop and execute a program of environmental education in the community, with the objective of eliminating the practice of dumping trash on the river and its tributaries.