An Assessment of Stream Water Quality of the Rio San Juan, Nuevo León, México, 1995-1996 - 2002

7/29/2019 An Assessment of Stream Water Quality of the Rio San Juan, Nuevo Len, Mxico, 1995-1996 - 2002

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Surface Water Quality

An Assessment of Stream Water Quality of the Rio San Juan,Nuevo Leon, Mexico, 19951996

Jose Santos Flores Laureano and Jose Navar*

ABSTRACT San Juan watershed as the third most polluted in thecountry. The European Union (Commission of the Eu-Good water quality of the Rio San Juan is critical for economicropean Community, unpublished data, 1991) proposeddevelopment of northeastern Mexico. However, water quality of the

river has rapidly degraded during the last few decades. Societal con- a management plan for all reservoirs of the Rio Sancerns include indications of contamination problems and increased Juan watershed due to eutrophication problems fromwater diversions for agriculture, residential, and industrial water sup- high inputs of organic matter in the main stem of theplies. Eight sampling sites were selected along the river where water Rio San Juan. In response, the government of Nuevosamples were collected monthly for 10 mo (October 1995July 1996). Leon initiated a sanitation program in 1994 entitled PlanThe concentration of heavy metals and chemical constituents and

Monterrey IV, which included the construction of threemeasurements of bacteriological and physical parameters were deter-

large wastewater treatment plants and the discharge ofmined on water samples. In addition, river discharge was recorded.municipal effluents and treated water to other Rio SanConstituent concentrations in 18.7% of all samples exceeded at least

Juan tributaries. However, even after the aforemen-one water quality standard. In particular, concentrations of fecal andtotal coliform bacteria, sulfate, detergent, dissolved solids, Al, Ba, tioned tasks were completed, the public perception wasCr, Fe, and Cd, exceeded several water quality standards. Pollution that pollution problems persisted within the Rio Sanshowed spatial and temporal variations and trends. These variations Juan.were statistically explained by spatial and temporal changes of constit- Heavy metal pollution in the Rio San Juan is a majoruent inputs and discharge. Samples collected from the site upstream concern in the Rio Bravo as well (Schmandt et al., 2000;of El Cuchillo reservoir had large constituent concentrations when

United StatesMexico International Boundary and Wa-discharge was small; this reservoir supplies domestic and industrial

ter Commission, 1994), the Rio Blanco in Veracruz,water to the city of Monterrey.Mexico (Albert and Badillo, 1986), the Rio Tijuana inBaja California, Mexico (Gersberg and Trindale, 1989),and the Rio Coatzacoalcos in Veracruz, Mexico (Toledo

The Rio San Juan watershed is located within a et al., 1989).semiarid region between the physiographic prov-Few water quality studies have been conducted withininces of the Great Plains of North America, the Chihua-

the Rio San Juan watershed. The National Water Com-huan Desert, and the Northern Plains of the Gulf of

mission, the official federal government agency respon-Mexico. Availability of water resources within the area sible for assessing water quality in Mexico, routinelyis highly variable due to low, periodic precipitation,monitors some physical, chemical, and bacteriologicalrecurrent drought episodes, and high evapotranspira-parameters, but the monitoring program does not in-tion rates (Navar et al., 1994; Navar, 1999a). In orderclude the evaluation of heavy metals, pesticides, andto meet agricultural, industrial, and residential waterother organic compounds. Kramar et al. (1992) evalu-demands, several reservoirs (Marte R. Gomez, El Cuch-ated the concentration of several heavy metals in sedi-illo, and La Boca) have been built along the headwatersments of the Rio Santa Catarina, an important tributaryof the major stem of the Rio San Juan. The Rio Sanof the Rio San Juan, and described contamination byJuan is the major tributary of the lower Rio BravoRioFe, Cu, Zn, Cd, and Sr. Vogel et al. (1995) assessedGrande, which runs along the border between Mexicothe pollution of the Rio Pesqueria, another importantand the USA, from Ciudad Juarez (Chihuahua) and Eltributary of the Rio San Juan, and reported that Be,Paso (Texas) to its outlet into the Gulf of Mexico nearNi, Pb, Cd, and Sb exceeded the Mexican water qualityMatamoros (Tamaulipas) and Brownsville (Texas).standards for drinking water.Availability of water resources has been impaired by

The objectives of this current (19951996) study werecontamination from industrial and residential sourcesto (i) assess the water quality of the Rio San Juan andof the Rio San Juan. In 1988, the Ministry of Urban(ii) analyze the temporal and spatial variations of 22Development and Ecology (SEDUE) classified the Rioheavy metals and 23 physical, chemical, and bacteriolog-ical water quality parameters of the Rio San Juan and

J.S. Flores, Faculty of Forestry, University of Toronto, 33 Wilcocks the tributary Rio Santa Catarina. Emphasis was placedSt., Toronto, ON, Canada M5S 3B3. J. Navar, Facultad de CienciasForestales, UANL, Carr Nacional, Km 145, Linares, Nuevo Leon,Mexico, CP 67700. Received 11 June 2001. *Corresponding author

Abbreviations: ANOVA, analysis of variance; BOD, biological oxy-([email protected]).gen demand; COD, chemical oxygen demand; PCA, principal compo-nent analysis.Published in J. Environ. Qual. 31:12561265 (2002).

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FLORES & NAVAR: STREAM WATER QUALITY OF THE RIO SAN JUAN, MEXICO 1257

Fig. 1. The location of the Rio San Juan watershed and the water quality monitoring stations along the Santa Catarina and San Juan rivers.

the eastern suburbs of the city of Monterrey to its junctionon heavy metal pollution given the lack of informationwith the Rio San Juan; and the Rio San Juan from below Lain this respect.

Boca reservoir to El Cuchillo reservoir in the municipality ofChina, Nuevo Leon, 102 km to the east of Monterrey.

MATERIALS AND METHODS

Study Area Methodology

A total of eight river sampling sites were selected for streamThe Rio San Juan watershed covers an area of 33 000 km2

within the Mexican states of Cohauila, Nuevo Leon, and Ta- water collection and analysis: three sites were selected alongthe Rio Santa Catarina and five additional sites were locatedmaulipas in the northeastern portion of the country. Most of

the watershed area (57%) is located in Nuevo Leon. The Rio along the Rio San Juan, between La Boca and El Cuchilloreservoirs (Fig. 1).San Juan originates in the Sierra Madre Oriental mountain

range approximately 30 km to the southeast of the city of Twenty-two heavy metals were evaluated: Ag, Al, As, B,Ba, Be, Cd, Co, Cr, Cu, Fe, Hg, Li, Mn, Mo, Ni, Pb, Sb, Se,Monterrey, and flows in a northeasterly direction through the

Great Plains of North America and the Northern Plains of Si, Sn, and Zn, and two major ions, Ca and Mg. In addition,23 physical, chemical, and bacteriological water quality param-the Gulf of Mexico (Fig. 1). The river drains into the Rio

Bravo near Camargo in the state of Tamaulipas. eters were assessed: color, electrical conductivity, dissolvedsolids, suspended solids, total solids, turbidity, alkalinity, chlo-The climate of the Rio San Juan watershed area ranges

from semiarid to arid. Monthly rainfall has a bimodal type of ride, biological oxygen demand (BOD), chemical oxygen de-mand (COD), detergents, oil content, total P, total hardness,distribution, with the first peak occurring in MayJune and

the second peak, the most important in terms of total depth, nitrate as N, nitrite as N, ammonia as N, dissolved oxygen,pH, sulfate, fecal coliform bacteria, total coliform bacteria,occurring during SeptemberOctober. Long-term mean an-

nual precipitation (19351996) ranges from 250 to 1300 mm and temperature.Heavy metal analysis consisted of three different methodsyr1 (Navar, 1999a). The eastern slopes of the Sierra Madre

Oriental mountain range receive up to 1600 mm yr1, whereas of atomic absorption and emission. A PerkinElmer (Wellesley,MA)Zeeman5100 spectrophotometer equipped witha graphitethe western portion of the watershed, located in the Chihua-

huan Desert, receives as little as 200 mm yr1. furnace was used to determine Pb. A Beckman (Fullerton, CA)Model 1272 spectrophotometer with hydride generation wasThe Rio San Juan watershed houses a total population of

approximately 5 million inhabitants (Navar, 1999b), including used to measure the concentration of As, Hg, and Se. Theremaining heavy metals were determined by the method ofthe metropolitan areas of Saltillo and Monterrey (Fig. 1). The

study focused only on the tributary Rio Santa Catarina from atomic emission by argon plasma using a Thermo Jarrell Ash


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1258 J. ENVIRON. QUAL., VOL. 31, JULYAUGUST 2002

(Franklin, MA) PolyScan 61 spectrophotometer. For chemical sampling sites for all dates sampled. The probability of ex-ceedance for heavy metals was calculated by using the cumula-constituents and physical and bacteriological parameters the

laboratory methods used were those of the National Water tive distribution, according to Eq. [1] (Haan, 1986):Commission (Ministry of Urban Development and Ecology,1990) denominated the NMX-AA-XX (method number)-YY P (x X) 1 expx

[1](year) protocols for water analysis; for example, the methodused for alkalinity determination is NMX-AA-36-1980. where p(x ) probability of the random variable x, less than

Monthly water samples were collected at each of the eight or equal to x. Parameters , , and are shape, scale, andsites for a 10-mo period from October 1995 to July 1996. location parameters, respectively, which were estimated byA total of 80 stream water samples were analyzed for each the conventional procedure of moments. Haan (1986) re-parameter. However, due to several problems, the number of ported that the skew coefficient () is related to the shapesamples collected for fecal coliform bacteria, total coliform parameter () by Eq. [2]:bacteria, detergents, and nitrate was 56, 72, 72, and 64, respec-tively.

Water samples were collected at midstream width by way

(1 3/) 3 (1 2/)

(1 1/) 23 (1 1/)

[ (1 2/) 2 (1 1/)]3/2[2]of wading and submerging plastic containers with a volume

of 4 L to a depth of 20 to 30cm. Samples analyzed for dissolvedand with this he mathematically defined and by Eq. [3]oxygen were collected and stored in 300-mL Winkler bottles,and [4]:and preserved with 2 mL of manganous sulfate and 2 mL of

an alkali iodide azide of sodium solution to fix the oxygen.Water samples for bacteriological analyses were collected in

2

(1 2/) 2 (1 1/)1/2

[3]125-mL sterile glass bottles. Water samples for oil contentanalysis were collected in 1-L glass bottles previously washed

(1 1/) [4]with hexane, and later preserved with 2 mL of hydrochloricacid. Water samples for heavy metal analysis were collected where and are the average and standard deviation ofin 1-L acid-rinsed polyethylene containers and preserved with the random variable, respectively. The shape parameter is3 mL of nitric acid. iteratively fitted by estimating first the skew coefficient in Eq.

Discharge (Q ) was estimated by measuring stream velocity [3] to later solve for and .(V) and cross-sectional area (A ) of the river channel during Mass flux (mg s1 ) for each site and sampling event waseach sampling event. Velocity was measured using a cup-type calculated by multiplying the monthly concentration of eachcurrent meter by counting the rotors number of revolutions heavy metal (mg L1 ) times discharge (L s1 ). These calcula-during a measured time interval. Velocity was then estimated tions were used to compare spatial and temporal behavior of

each heavy metal among sampling sites. The mass flux of allby using a rating table,which relates the numberof revolutionsheavy metals was added together to obtain a single value forand time of measurements. The cross-sectional area of theeach site and date of sampling (total mass flux). Mass fluxriver was divided in smaller, discrete areas, where velocity waswas used to compare heavy metal concentrations among sitesrecorded. Cross-sectional areas were calculated by multiplyingand sampling events and to establish other plausible explana-width, usually 2 m, and average water depth, measured at thetions of pollution dynamics in the river system.two extreme sides of each segment. Total river discharge (Q

Principal component analysis (PCA) is a multivariate ordi-VA) was calculated by adding the partial discharges of eachnation technique used to extract information on patterns andsegment and is referred to as monthly instantaneous discharge.clusters of data (Ter andSmilauer, 1998).Principal componentanalysis has been successfully tested to provide reasonable

Statistical Analysis representation of water chemistry variables in the Cache laPoudre River in Colorado (Shieh et al., 1999). The waterConstituent concentrations were compared with Europeanchemistrydata of total mass flux of heavy metals wasemployedCommunity standards (Tebutt, 1994),World HealthOrganiza-to detect whether sites andsampling events showed tendenciestion standards (Tebutt, 1994), and official Mexican standardsor clustering patterns. The PCA program was fed with matrix(Secretaria de Salubridad y Asistencia, 1996) for domesticdata composed of 22 heavy metals and 80 samplings: 8 siteswater supply, and the Mexican Water Commission standardsand 10 events. In addition, the total mass flux of heavy metals(Ministry of Urban Development and Ecology, 1990) for eco-was statistically analyzed using analysis of variance (ANOVA)logical criteria. A total of 11 696 comparisons, which werewith sampling sites and events as the major sources of varia-estimated by multiplying the number of laboratory analysestion. Multiple comparisons were conducted using Tukey statis-for each parameter by the number of standards available fortical tests. The ANOVA and Tukey analysis provided resultseach parameter, were used for data analysis. These compari-for each constituent (45) by sampling site and event. There-sons allowed the estimation of the number of exceedances for

fore, the discussion centers the attention on the conclusionseach sampling event, which in turn were used to establishof this analysis rather than on reporting statistical results forspatial and temporal trends among sites and sampling events.each constituent. Finally, plots of total mass and standardizedIn addition, the relationship between discharge and concentra-concentrations of heavy metals against instantaneous monthlytion of exceeding parameters or constituents was analyzed.discharge were observed to describe changes in heavy metalContinuous dispersion of constituents was assessed by fit-concentrations with changes in monthly instantaneous dis-ting the Weibull distribution to the concentration of heavycharge.metals and chemical constituents, which exceeded the water

quality standards. The Weibull distribution has also been usedRESULTS AND DISCUSSIONto evaluate dispersion mechanisms of odor pollution (Pringer

and Schauberger, 1999) as well as other processes such as Concentration Analysisthe lifetime of Escherichia coli (Hutchinson, 2000) and the

Of all evaluated parameters only 40 could be com-dispersion of fluvial gravels (Kondolf and Adhikari, 2000).The distribution was fitted to the data generated at all eight pared with water quality standards. There are no stan-


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Table 1. Mean heavy metal and chemical constituent concentrations and measurements of physical and bacteriological parameters thatexceeded standards in water samples collected in the Rio San Juan and the Rio Santa Catarina during 1995 and 1996.

Parameter or constituent NOM WHO EC WR AGR# LC PAB

%

Al 57.0 57.5 NA 100.0 0.0 0.0 98.8Sb NA NA NA 7.5 7.5 NA 7.5As 6.3 6.3 7.5 6.3 6.3 6.3 6.3Ba 0.0 NA 7.5 0.0 NA NA 98.8

Cd 37.5 37.5 87.5 1.3 1.3 0.0 NACu 0.0 0.0 3.8 0.0 0.0 0.0 NACr 1.3 1.3 1.3 1.3 0.0 0.0 93.8Fe 38.8 38.8 87.5 38.7 0.0 NA 3.8Mn 5.0 17.5 31.3 17.5 NA NA NANi NA 0.0 NA 77.5 0.0 0.0 NAAg NA NA NA 2.5 NA NA NAPb 1.3 1.3 1.3 1.3 0.0 0.0 NAZn 0.0 0.0 2.5 0.0 0.0 0.0 NAColor 76.3 83.8 90.0 21.3 NA NA NAElectrical conductivity NA NA NA 20.0 NA NA NADissolved solids 7.5 7.5 NA 92.5 92.5 7.5 NASuspended solids NA NA 17.5 0.0 10.0 NA NATotal solids NA NA NA 11.3 NA NA NATurbidity 60.0 60.0 NA NA NA NA NABOD NA NA 35.0 NA NA NA NACOD## NA NA 11.3 NA NA NA NADetergents 0.0 NA 4.2 0.0 NA NA 28.7Oil content NA NA NA 82.5 NA NA NA

Total phosphorus NA NA 1.3 41.3 NA NA NATotal hardness 7.5 7.5 NA NA NA NA NANitrate as N 1.6 1.6 0.0 12.5 NA 0.0 NANitrite as N 2.5 0.0 NA 2.5 NA 0.0 NAAmmonia as N 10.0 NA 45.0 NA NA NA 25.0Dissolved oxygen NA NA NA 0.0 NA NA 3.8pH 1.3 1.3 1.3 0.0 0.0 NA NASulfate 6.3 NA 76.3 6.3 85.0 NA 100.0Fecal coliform bacteria 100.0 100.0 69.6 26.8 26.8 NA 50.0Total coliform bacteria 97.2 100.0 81.9 NA NA NA NA

Official Mexican standard for drinking water. World Health Organization standard for drinking water. European Community standard for drinking water. Ecological criteria of the National Water Commission for water reservoirs.# Ecological criteria of the National Water Commission for agriculture. Ecological criteria of the National Water Commission for livestock consumption. Ecological criteria of the National Water Commission for protection of aquatic biota. Not available. Biological oxygen demand.## Chemical oxygen demand.

dards for Ca, Co, Li, Mo, Si, Sn, and Mg. In addition, ids, detergents, oil content, total P, nitrate as N, nitriteas N, ammonia as N, dissolved oxygen, sulfate, fecalonly 14 heavy metals could be statistically analyzed,

since Be, Hg, and Se recorded concentrations were be- coliform bacteria, and total coliform bacteria. For theecological criteria, a few constituents exceeded at leastlow limits of detection. Therefore, only 11 696 compari-

sons could be made, of which 2190 (18.7%) exceeded one standard for agriculture (Sb, As, Cd, dissolved sol-ids, suspended solids, sulfate, fecal coliform bacteria,the standards. Table 1 shows the mean values of the

constituents and parameters that exceeded most of the and total coliform bacteria) and for livestock consump-tion (As and dissolved solids exceeded the standards inwater quality standards. Concentrations of Al, Ba, Cd,

Cr, Fe, Mn, and Ni exceeded the standards in more less of 10% of the samples). Aluminum, Sb, As, Cd, Cr,Fe, Mn, Ni, Ag, color, electrical conductivity, dissolvedsamples than Ag, As, Cu, Sb, Pb, and Zn. Dissolved

solids and color exceeded standards in more samples solids, total solids, oil content, total P, nitrate as N,nitrite as N, sulfate, and fecal coliform bacteria exceededthan electrical conductivity, suspended solids, and tur-

bidity. Sulfate concentrations exceeded standards in more the National Water Commission standards for waterin reservoirs.samples than nitrate and nitrite concentrations, and pH

(Table 1). The standards for protection of the aquatic biota wereexceeded by the concentration of Al, Sb, As, Ba, Cr,Constituents exceeding at least one standard for

drinking water were Al, As, Ba, Cd, Cu, Cr, Fe, Mn, Pb, Fe, detergents, ammonia as N, dissolved oxygen, sulfate,and fecal coliform bacteria.Zn, color, dissolved solids, suspended solids, turbidity,

BOD, COD, detergents, total P, total hardness, nitrate Table 2 shows several statistics of the constituentsand parameters that exceeded most of the water qualityas N, nitrite as N, ammonia as N, pH, sulfate, fecal

coliform bacteria, and total coliform bacteria. For the standards. The Commission of the European Commu-nity (unpublished data, 1991) recorded, for three sam-ecological criteria, parameters exceeding the standards

were Al, Sb, As, Ba, Cd, Cr, Fe, Mn, Ni, Ag, Pb, color, ples collected between 1985 and 1990 near Site 3 of thisstudy, a P concentration of 1.5, 4.5, and 7.9 mg L1 andelectric conductivity, dissolved solids, sulfate, total sol-


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Table 2. Concentration statistics forselected constituents and parameters that exceeded water quality standardsin water samplescollectedin the Rio San Juan and Rio Santa Catarina in 1995 and 1996. Standards between brackets are in units in mg L 1, unless a differentunit is specified.

Constituent or parameter Mean Standard deviation Skew coefficient

Al (0.2, 5.0#, 0.05) 0.517 0.735 2.87Ba (0.7, 0.1, 1.0, 0.01) 0.067 0.025 1.34Cd (0.005, 0.001, 0.01#, 0.02) 0.005 0.002 0.12Cr (0.05, 1.0#,0.01) 0.021 0.008 1.31

Fe (0.3, 0.1, 5.0#) 0.362 0.389 3.76Mn (0.15, 0.1, 0.05) 0.067 0.125 5.13Ni (0.1, 0.01, 0.2#, 1.0) 0.015 0.006 0.44Color (color units; 20.0, 15.0, 10.0, 75.0) 59.6 68.3 3.30Electrical conductivity (S m1; 1000.0) 8825 2335 1.18Biological oxygen demand (3.0) 3.4 2.3 2.14Chemical oxygen demand (30.0) 12.5 12.4 2.61Total phosphorus (0.4, 0.1) 0.105 0.078 1.45Oil content (absent) 2.5 2.6 1.34Nitrate as N (10.0, 25.0, 5.0, 90.0) 1.6 2.3 2.17Ammonia as N (0.5, 0.05, 0.06) 0.152 0.201 2.32Dissolved solids (1000.0, 500.0#) 712.4 192.0 0.80Suspended solids (25.0, 500.0, 50.0#) 20.2 27.9 3.07Total Solids (1000.0) 732.6 192.5 0.77Sulfate (400.0, 150.0, 500.0, 130.0#, 0.005) 211.3 116.1 2.12Turbidity (itu; 5.0) 12.9 19.5 4.02Fecal coliform bacteria (coliform per 100 mL; 0, 20, 1000, 100#, 200) 111 546.0 674 282.0 7.20Total coliform bacteria (coliform per 100 mL; 2, 0, 50) 332 679.7 1 971 163.2 7.47

Official Mexican standard for drinking water. World Health Organization standard for drinking water. European Community standard for drinking water. Ecological criteria of the National Water Commission for water reservoirs.# Ecological criteria of the National Water Commission for agriculture. Ecological criteria of the National Water Commission for livestock consumption. Ecological criteria of the National Water Commission for protection of aquatic biota.

biological oxygen demand of 107.9, 19.3, and 5.0 mg standards, but suggested that construction of El CuchilloL1. The 1991 research determined a mean P concentra- reservoir (closed in 1993) could deteriorate streamwatertion of 0.105 and a biological oxygen demand of 3.4 quality, which appears to be confirmed by this analysis.mg L1, which indicated that the Plan Monterrey IV Because the Rio Ramos and Rio Pilon tributaries diluteimproved but did not totally eradicate contamination some of the contamination of the main stem of the Rioproblems in the Rio San Juan watershed. San Juan and land use affects water quality upstream

The number of constituent analyses and parameter from the confluence of major tributaries, it is likely that

measurements that exceeded water quality standards backwater is having a deleterious effect on stream watervaried among sites (Fig. 2). The increasing frequency quality at Site 8 and possibly Site 7.in the number of exceeded constituent analyses and The number of constituent analyses exceeding waterparameter measurements from Site 1 to 4 appears to quality standards generally increased from 77 in Octo-be related to increasing residential and industrial land ber 1995 to 144 in July 1996 (Fig. 3). However, monthlyuse between the Rio Santa Catarina and the Rio SanJuan. Site 5, located downstream of the confluence ofthe Rio San Juan and the Rio Ramos, implies a dilutioneffect. The Rio Ramos originates in the Sierra MadreOriental mountain range and flows through severalsmall villages, where dominant land use is agricultureand industrial activity is low, resulting in the dilution ofpollutant concentrations at Sites 5 and 6.

The frequency of constituent analysis and parameter

measurements surpassing water quality standards in-creased downstream from Site 6 to 8. Site 8 was locatedimmediately downstream from the confluence of theRio Pilon and Rio San Juan. The Rio Pilon, like theRio Ramos, drains through landscapes dominated byagriculture with sparse industrial activity in the basin.Land use activities associated with urban centers andindustry along the major stem of the Rio San Juan arecontributing to increased pollution at Sites 7 and 8. TheCommission of the European Community (unpublished

Fig. 2. The number of water quality exceedances at sampling sitesdata, 1991) found that a few kilometers downstream of along the Rio San Juan and the Rio Santa Catarina in Nuevo Leon,

Mexico in 1995 and 1996.Site 8, streamflow samples met several water quality


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variations in exceedances are evident. For example, more important at several locations along the river orat different times during the sampling period.samples collected in November 1995 and January 1996

The relation between concentration of constituentsrecorded 114 and 75 element concentrations exceeding(Al, Ba, Cd, Cr, Fe, Mn, and Ni) and instantaneouswater quality standards, respectively. Considering themonthly discharge did not show clear trends. Due toseasonality of streamflow and assuming a steady, con-the severe drought that occurred during the period ofstant reduction of discharge of wastewater effluent intostudy (only isolated, convective storms occurred at sev-the river, other point and nonpoint sources of contami-

eral locations in the watershed), instantaneous monthlynation may be affecting the river at specific time inter-discharge showed a consistent, logarithmic reduction invals at specific locations.volume. Assuming a steady input of heavy metals andIn total, 15 heavy metal constituents (Al, Sb, Ba, B,constituents into the river and given the reduction ofCd, Cu, Cr, Sn, Fe, Li, Mg, Mn, Ni, Ag, and Zn) anddischarge, we could expect an increase in constituent14 chemical constituents and physical parameters (alka-concentrations over time. However, some parameterslinity, chloride, sulfate, total hardness, phosphate, ni-(Al, Fe, Mn, turbidity, color, suspended solids, electri-trite, dissolved oxygen, pH, dissolved solids, suspendedcal conductivity, BOD, COD, ammonia, and sulfate)solids, total solids, color, electrical conductivity, andshowed an increase in concentration as instantaneousturbidity) fit the Weibull distribution (p 0.05). Thismonthly discharge diminished through time, while manytendency partially explains the diffusion process of theseothers such as Ba, Cr, Cd, Ni, dissolved solids, totalconstituents and parameters among sites and samplingsolids, phosphate, and oil showed an erratic behavior,events along the main stem of the Rio San Juan andwith alternating peaks and valleys through the studythe tributary Rio Santa Catarina. Using the Weibullperiod. This observation demonstrates the randomnessdistribution, the probability of exceedance for Al wasof the point-source pollution phenomenon in this water-0.53 and 0.88 (official Mexican standards and the ecolog-shed. Iron and Mn showed consistent patterns of decayical criteria of the National Water Commission for pro-with increasing instantaneous monthly discharge, indi-tection of aquatic biota, respectively), Ba was 0.99 (eco-cating the dilution effect of discharge and the steadylogical criteria of the National Water Commission forinput of these constituents into the river.

protection of aquatic biota), Cd was 0.56 (Mexican stan-dards), Cr was 0.93 (ecological criteria of the National

Mass AnalysisWater Commission for protection of aquatic biota), Mnwas 0.73 and 0.84 (Mexican standards and the ecological The results of the principal component analysis, PCA,criteria of the National Water Commission for livestock conducted on the heavy metal mass fluxes indicated thatconsumption, respectively), and Ni was 0.92 (ecological the first two eigenvalues explained 93% of the totalcriteria of the National Water Commission for livestock variation. A plot of the first two eigenvectors showedconsumption). Point-source pollution probably played two different groups, characterized by the relative metalan important role in contaminating the headwaters of mass flux. Cluster 1 was characterized by Sites 2 and 3,

the Rio San Juan at several locations and at several and Cluster 2 by Sites 1, 4, 5, 6, 7, and 8 (Fig. 4). Thetimes throughout the sampling period, and because of first component appeared to distinguish sampling sitesthe diffusion process, pollution was present down- associated with the nature of local point-source pollu-stream. Inputs of the remaining constituents and param- tion. The second component explained a small amounteters that did not fit the Weibull distribution may be

Fig. 4. The results of the principal component analysis conducted onFig. 3. The number of water quality exceedances at monthly samplingsites along the Rio San Juan and the Rio Santa Catarina in Nuevo the heavy metal mass fluxes for eight sampling sites located in the

Rio San Juan of Nuevo Leon, Mexico.Leon, Mexico in 1995 and 1996.


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Table 3. Results of a randomized block analysis of variance (ANOVA) design for constituent analysis between sampling sites (8) andsampling events (10) for the mass flux of 21 heavy metals in the Rio San Juan of Nuevo Leon, Mexico during 19951996.

Results of a randomized block ANOVA design

Model Sampling SamplingHeavy metal CV Mean F P F sites (P F) events (P F)

mg s1

Al 161.25 226.08 4.51 0.0001 0.1001 0.0001

Sb 110.77 29.24 6.07 0.0001 0.0055 0.0001As 783.85 11.65 1.15 0.3286 0.4325 0.2731Ba 91.69 38.51 7.42 0.0001 0.0023 0.0001B 75.09 103.82 9.45 0.0001 0.0001 0.0001Cd 90.85 2.89 7.71 0.0001 0.0010 0.0001Ca 95.94 90 403.14 6.60 0.0001 0.0487 0.0001Co 118.68 1.72 5.16 0.0001 0.0079 0.0001Cu 130.76 5.72 6.46 0.0001 0.1250 0.0001Cr 87.31 13.62 7.99 0.0001 0.0027 0.0001Sn 91.53 28.96 7.95 0.0001 0.0004 0.0001Fe 224.43 183.00 2.88 0.0014 0.1924 0.0005Li 89.51 11.26 7.00 0.0001 0.0001 0.0001Mg 109.03 10 765.28 5.51 0.0001 0.0024 0.0001Mn 160.40 16.53 3.37 0.0003 0.1382 0.0001Mo 96.12 5.42 6.35 0.0001 0.0038 0.0001Ni 96.52 8.25 6.63 0.0001 0.0056 0.0001Ag 151.58 6.36 7.69 0.0001 0.0309 0.0001Pb 257.13 1.69 1.45 0.1466 0.2436 0.1546Si 121.92 4 780.94 5.90 0.0001 0.0264 0.0001Zn 315.94 26.74 1.47 0.1417 0.4120 0.0865

of variation and could be related to nonpoint-sourcepol- The statistical test confirms that the total mass of heavymetals decreased from October 1995 to July 1996. Thelution.

Clusters formed by the PCA analysis were partially ANOVA and Tukey tests also showed that the highestconstituent means of total mass fluxes corresponded toconfirmed by the ANOVA and Tukey tests, where Sites

1, 7, and 8, recorded the largest mean mass flux and Dates 1, 2, 3, and 4 and the lowest to Dates 8, 9, and10 for most heavy metals (p 0.05). The reduction wasSites 2, 3, and 4 recorded the smallest mean mass flux

(p 0.05). The Tukey test ranked Site 8 with the highest partially explained by the reduction of instantaneousaverage monthly discharge reported from October 1995mean mass flux values for 14 out of 21 heavy metals.

This site recorded the highest mean monthly discharge (2472.0 L s1 ) to July 1996 (60.0 L s1 ).The analysis of variance showed statistical differencesand the second-highest constituent concentration. How-

ever, Sites 5 and 6 had the smallest frequency of ex- among sampling events for Ag, B, Ba, Ca, Cd, Co, Cr,Li, Mg, Mo, Ni, Sb, Si, and Sn, confirming the point-ceedances but they had one of the largest estimates of

instantaneous monthly discharge in the study area. This source pollution theory, where the seasonality of efflu-ent discharge exceeded set standards. On the otherinformation confirms the original dilution effect theory

of the tributaries Rio Ramos and Rio Pilon. hand, Al, Cu, Fe, and Mn only showed statistical differ-ences among sampling events, emphasizing that only theThe ANOVA test indicated that only seven heavy

metals (Al, As, Cu, Fe, Mn, Pb, and Zn) did not showstatistical differences between sampling sites (p 0.05).Aluminum, Fe, and Mn frequently exceeded set stan-dards indicating the widespread presence of these con-stituents along the Rio San Juan (Table 3). The 14 heavymetals that showed statistical differences between sam-pling sites rarely exceeded set standards, with the excep-tion of Cd.

The PCA for the temporal analysis of heavy metalmass flux also indicated that the first two eigenvalues

explained most of the total variation (86%). Therefore,a graph of the first two eigenvectors showed a weakclustering but a strong tendency with October 1995 (T1)first and July 1996 (T10) last (Fig. 5). The first PCAdivides sampling events into two different seasons inheavy metal mass flux. The wet season includes Sam-pling Events 1 to 5 and the dry season includes SamplingEvents 6 to 10. Therefore, this component can be namedthe seasonal input of constituents. The second PCAappears to comprise the variation explained by changes

Fig. 5. The results of the principal component analysis conducted onin instantaneous monthly discharge associated with the the heavy metal mass fluxes for 10 sampling events observed in

the Rio San Juan of Nuevo Leon, Mexico in 19951996.within-season variations of constituents into the river.


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temporal discharge of effluents exceeded set standards. concentration in October 1995 was 0.584 mg L1 butthemean mass flux reached itsmaximum value of 1004.6Finally, As, Pb, and Zn did not show statistical differ-

ences either among sites or sampling events (Table 3). mg s1. Consequently, an increased pollutant load wasdelivered to the El Cuchillo reservoir during the rainyA positive nonlinear relationship between total mass

flux of heavy metals and average instantaneous monthly season. Averaging the mass flux of all 80 samples foreach heavy metal and using this value to calculate thedischarge was found for all monitoring sites (Fig. 6). The

increase of average instantaneous monthly discharge total masses, it is possible to estimate the load of metals

transported by the river in Mg yr1. Estimated metalimmediately during or after the rainy season, combinedwith the increase in discharge from sewage effluent, loads in the Rio San Juan were approximately 7.1 for

Al, 1.2 for Ba, 0.09 for Cd, 0.4 for Cr, 5.8 for Fe, 0.5wastewater, and other industrial sources into the RioSan Juan appears to be responsible for the increase in for Mn, and 0.3 Mg yr1 for Ni. Thus, a water quality

concern persists throughout the year with high concen-total mass of heavy metals transported by the river.Total mass of heavy metals increases steeply with small trations of some constituents during the dry season and

high load input into El Cuchillo reservoir during thechanges in the rivers average instantaneous monthlydischarge. However, the relationship attained a steady rainy season.

The Rio San Juan watershed is located in an areastate condition when average instantaneous monthlydischarge was approximately 1000 L s1. The total mass prone to recurrent drought episodes of different magni-

tudes and time scales. Navar (1999a,b) observed thatof heavy metals became limited when the average in-stantaneous monthly discharge of the river was larger the 1950s, the early 1980s, and the 1990s were character-

ized by below-average monthly rainfall and dischargethan 1000 L s1 (Fig. 6).The water quality in the Rio San Juan and the Rio at several gauging and climatic stations along the Rio

San Juan. Schmandt et al. (1998, 2000) concluded thatSanta Catarina was assessed in this study by using twocontrasting approaches: the analysis of concentrations the drought during the 1990s has not ended in the lower

Rio Bravo watershed. In droughts, such as the presentand the analysis of total masses of pollutants. The con-centration approach showed that during the dry season one, water supplies needed for irrigation of agriculture

of the Rio San Juan watershed (170 000 ha) demandedsome constituents and parameters had large concentra-tions that exceeded standards, and consequently jeop- 1680 Mm3 yr1, in contrast to 1200 Mm3 yr1 during

normal years (Navar and Rodrguez, 2002). Futureardized the health of the rivers aquatic ecosystem. Us-ing the total mass analysis approach the concentration drought episodes will probably worsen the concentra-

tion of several constituents and parameters, deteriorat-of some constituents and parameters typically decreasedconsiderably during the rainy season. However, the total ing water quality and further impairing water availabil-

ity in the Rio San Juan watershed.mass of material transported is significantly greater dur-ing this period due to (i) the washing effect that rainfall The population of the Monterrey metropolitan area

(MMA) in 1995 was 3.8 million inhabitants (Navar,has over the entire watershed, especially in densely pop-ulated areas and (ii) the increased input of sewage efflu- 1999a), who have used 201 Mm3 yr1 of domestic water

supply. Population in the MMA is expected to increaseents, wastewater, and other industrial discharges. Forexample, the Al concentration reached its highest con- to 6.3 million by the year 2020 (Consejo Nacional de

Poblacion, 1996), with demands for domestic water esti-centration mean value in July 1996 (2.356 mg L1 ) andthe mean mass flux was 96.5 mg s1, whereas the mean mated at 374 Mm3 yr1 by 2020 and 545 Mm3 yr1 by

2045 (Navar, 1999a). De la Garza (1995) observed that89% of discharges to the Rio San Juan by the 10 000industries in the Rio San Juan watershed exceeded thestandards. Therefore, degradation of water quality inthe Rio San Juan watershed may involve two factors: (i)increasing water withdrawal demands to meet domesticand industrial water supplies and (ii) increasing loadsof pollutants into the river.

The aquatic communities of fish, insects, and otherorganisms are already under stress. It was observed, in

this study, that many parameters exceeded the ecologi-cal criteria of the National Water Commission for pro-tection of aquatic biota. For example, 72.5% of thesamples had P levels that exceeded the ecological crite-ria of the National Water Commission for protectionof aquatic biota. Brooks et al. (1992) pointed out that,if P is present in water, a concentration of 0.30 mg L1

of nitrate is enough to cause an increase in algae growth;64% of the samples exceeded thisthreshold value during

Fig. 6. Linear and nonlinear models for the total mass of 21 heavy 19951996. Therefore, the process of eutrophication canmetals for the sites and sampling events in the Rio San Juan of

be expected to accelerate depleting dissolved oxygenNuevo Leon, Mexico in 1995 and 1996. TM, total mass; Sx, stan-dard error. concentrations in the river and promote changes in the


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aquatic biota. The biochemical oxygen demand ex- ACKNOWLEDGMENTSceeded the European Community standard for drinking We are deeply grateful to the Direccion General de Educa-water in 35% of the samples. The susceptibility of fish cion Tecnologica Agropecuaria (DGETA) and the Consejoto damage by toxic substances (e.g., heavy metals) in- Nacional de Ciencia y Tecnologia (CONACYT) in Mexicocreases when the level of dissolved oxygen depletes City for the valuable support to develop this research. SIRE-

YES and PAICYT partially funded this research through(Brooks et al., 1992). Water temperature is another im-Grant Agreements no. 6033 and CN 323-00. The analyses ofportant factor controlling the oxygen solubility (Mc-

the physical, chemical, and bacteriological parameters wereDonald et al., 1991). Temperatures as high as 35C re- conducted within the water quality laboratory of the Nationalcorded in most sites in July 1996 during times of lowWater Commission in Monterrey. The Department of Phar-discharge may reduce the availability of dissolved oxy-macology of the Faculty of Medicine, University of Nuevo

gen and impair stream suitability for native fish species.Leon, conducted the analyses of heavy metals. We thank Dr.

The alkalinity of the stream waters combined with Alfredo Pineiro Lopez for his financial help for conductingsedimentation of heavy metals on the river bed or the this study.We express our gratitude to many of theanonymousbottom of the reservoirs under anaerobic conditions referees who greatly helped to improve the final text.may cause these constituents to become soluble andreenter the water column. This effect can be enhanced REFERENCESwith the presence of large runoff events that normally

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Documents

An Assessment of Stream Water Quality of the Rio San Juan, Nuevo León, México, 1995-1996 - 2002