TECNOCIENCIA CHIHUAHUA, Vol. XVIII (1) e 1415 (2024)

https://vocero.uach.mx/index.php/tecnociencia

ISSN-e: 2683-3360

Artículo Científico

Temporal evolution of nitrate in Meoqui-Delicias

aquifer in Chihuahua, Mexico

Evolución temporal de nitrato en el acuífero Meoqui-Delicias en

Chihuahua, México

*Correspondencia: Correo electrónico: mespino@uach.mx (María Socorro Espino-Valdés)

DOI: https://doi.org/10.54167/tch.v18i1.1415

Recibido: Fecha de recepción: 17 de noviembre de 2023; Aceptado: Fecha de aceptación: 15 de abril de 2024

Publicado por la Universidad Autónoma de Chihuahua, a través de la Dirección de Investigación y Posgrado.

Editor de Sección: Dr. Rubén Francisco González-Laredo

Abstract

The continued input of nitrate (NO3) into groundwater is a global problem, mainly associated to

excess fertilizer and improper disposal of human and livestock waste. Nitrate accumulation in oxic

aquifers of semiarid areas makes these zones especially susceptible to pollution. Nitrate in Meoqui-

Delicias aquifer, located in an important irrigation district in Chihuahua, Mexico, was quantified in

2021 in 63 drinking water wells. Samples collected were analyzed in laboratory and results were

compared to 2003 and 2006 data available for those wells. Nitrate values varied from 0.7 to 23.2 mg/L

and 22 % of the wells contained NO3 above drinking water guidelines (10 mg NO3-N/L). A low to

moderate nitrate pollution index (NPI) and a slight NO3-N variation with time was observed for most

wells. Values of NO3-N/Cl < 1.0 support an anthropogenic origin of nitrate. No association was found

between NO3-N and well depth. The most susceptible areas to nitrate contamination were identified

as those areas with high NO3-N and increasing concentration with time. The lack of a pattern of

contamination suggested leakage of manure leachate at a few points as the most likely contamination

source. The consistently high NO3-N content (>10 mg/L) in three deep wells constitutes a serious

concern.

Keywords: Nitrate pollution, irrigation, semiarid, water quality, manuere leachate, Chihuahua

María Socorro Espino-Valdés1*, Nayeli Villalobos-Gutiérrez1, Mélida Gutiérrez2,

Humberto Silva-Hidalgo1 y Adán Pinales-Munguía1

1 Universidad Autonoma de Chihuahua, Facultad de Ingenieria, Circuito Universitario, Campus II,

31124 Chihuahua, Chih. México

2 Missouri State University, Department of Geography, Geology and Planning, Springfield Missouri,

USA, 65897

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Resumen

La aportación continua de nitrato (NO3) en aguas subterráneas es un problema mundial, asociado

principalmente al exceso de fertilizantes y a la disposición inadecuada de desechos humanos y

ganaderos. En 2021 se cuantificó el nitrato en 63 pozos de agua potable del acuífero Meoqui-Delicias,

ubicado en un importante distrito de riego en Chihuahua, México. Las muestras colectadas fueron

analizadas en el laboratorio y los resultados se compararon con datos de 2003 y 2006 de los mismos

pozos. Los nitratos variaron de 0.7 a 23.2 mg/L; el 22 % de los pozos sobrepasaron la norma (10 mg

de NO3-N/L). Se observó un índice de contaminación por nitrato entre bajo y moderado y una ligera

variación del NO3-N con el tiempo para la mayoría de los pozos. Valores de NO3-N/Cl < 1.0 sugieren

origen antropogénico del nitrato. No se encontró relación entre NO3-N y la profundidad del pozo.

Las áreas de alto contenido de NO3-N y concentración creciente con el tiempo son las más susceptibles

a la contaminación. La falta de un patrón de contaminación sugiere que los residuos ganaderos

constituyen la fuente probable de contaminación en algunos puntos. El constante y alto contenido de

NO3-N (>10 mg/L) en tres pozos profundos constituye una preocupación.

Palabras clave: Contaminación por nitratos, irrigación, semiárida, calidad del agua, residuos

ganaderos, Chihuahua.

1. Introduction

The presence of groundwater nitrate (NO3) has been recognized as a global problem because of its

gradual increase in aquifers around the world (Galloway et al., 2008), its capacity to degrade drinking

water quality, and its ability to cause eutrophication to surface water bodies and to the ocean (Diaz

and Rosenberg, 2008; Hamlin et al., 2022). The main source of NO3 in groundwater has been traced

back to an excess of N-fertilizer applied to crops, either in the form of mineral N-fertilizer or as

manure (Galloway et al., 2008; Vitousek et al., 2009; Bijay and Craswell, 2021). Fertilizers are often

applied to crops in excessive amounts or out of the appropriate season (Gomes et al., 2023).

Nitrate (NO3) is a highly stable and soluble ion under the oxic and alkaline conditions that prevail in

aquifers in arid and semiarid areas (Re et al., 2021). Under these conditions, NO3 barely adsorbs or

precipitates, and thus accumulates in aquifers (Hansen et al., 2017; Xiao et al., 2022) where it may

remain in dissolved form for a long time (Gutiérrez et al., 2018; Mateo-Sagasta and Albers, 2018). In

arid and semiarid regions that have intensive agriculture and livestock farming a high content of NO3

is thus expected.

Since water quality of open aquifers under intense agricultural and livestock land use varies with

time, time series statistical methods have been a useful tool to determine and map its evolution and

to confirm statistically significant trends (Ducci et al., 2020; Hamlin et al., 2022). These methods

require a minimum of 8 to 10 measurements (e.g., years), an asset that many semiarid areas may not

have. Therefore, and in absence of a better alternative, information about the spatial NO3

concentrations and a rough estimation of their variation with time can be used to strategize water

management in agricultural and livestock areas lacking proper monitoring (Li et al., 2021), at least

until more data are gathered.

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Groundwater is the major source of drinking water in arid and semiarid areas. The gradual increase

of NO3 in the groundwater is an adverse finding since chronic ingestion of water containing high

levels of NO3 is harmful to humans, especially to infants under 6 months, who may develop

methemoglobinemia or cyanosis (Panneerselvam et al., 2020; Hamlin et al., 2022). The WHO limit for

drinking water is 10 mg NO3-N/L; however, threshold values lower than 10 mg NO3-N/L have been

proposed since human and environmental health complications starting at lower concentrations after

prolonged exposure, including 4.5 mg NO3-N/L (Panneerselvam et al., 2020) and 2 mg NO3-N/L

(Hamlin et al., 2022). Environmental impacts of NO3 include a major role in stimulating

phytoplankton growth, which may result in eutrophication and the development of hypoxic zones

in oceans (Diaz and Rosenberg, 2008). To curb these negative effects on the environment, a threshold

concentration of 3 mg NO3-N/L has been proposed (USEPA, 2007).

The N-cycle is complex, which makes the origin of NO3 difficult to pinpoint because of the many

possible natural or anthropogenic sources. Natural sources include atmospheric deposition and

dissolution of N-containing minerals. For most aquifers, these sources combined amount to a minor

contribution of the NO3 compared with its anthropogenic sources. As a result, NO3 is considered an

anthropogenic contaminant associated with an excess of N-fertilizer applied in agriculture (either

mineral fertilizer or manure) and domestic wastewater effluents (raw or treated wastewater and

septic tanks) (Mateo-Sagasta and Albers, 2018; Gutiérrez et al., 2022). A notable exception includes

arid regions where natural NO3 deposits form in the subsurface of arroyo floodplains (Walvoord et

al., 2003; Linhoff, 2022). These natural deposits have a ratio NO3-N/Cl > 1.5 and δ15N (NO3) < 8 ‰.

(Linhoff, 2022). These deposits have been observed in several desert areas around the world,

including parts of the Chihuahuan Desert (Walvoord et al., 2003).

Besides contributing to aquifer contamination and eutrophication of surface waters, NO3 from

agricultural non-point sources represents a loss of fertilizer resource and loss of profit to the farmer.

Therefore, management techniques that would minimize these losses are constantly sought (Rudolph

et al., 2015; Hansen et al., 2017; Sapkota et al., 2020). Another N compound generated in irrigated

agricultural fields is nitrous oxide (N2O), a potent greenhouse gas that is cumbersome to measure

and thus scarcely reported (Sapkota et al. 2020). However, the amount of N that becomes N2O is

generally a small fraction of the amount of NO3 formed (Millar et al., 2018).

If the content of NO3 and other N-compounds in surface waters and soil is high, plants will absorb

the amount they need, a small part will become gases, and most of the remaining N-compounds will

infiltrate through the soil and into the underlying aquifer. The infiltration rate is a complex process

that depends on many factors, including the soil type, amount of nitrogen in soils, recharge of the

aquifer, and depth to water table. Nitrate leakage into groundwater is faster and easier through soils

of high permeability (Gomes et al., 2023). Because of their high solubility, NO3 is easily carried by

infiltration water and becomes a common contaminant in oxic aquifers underlying agricultural and

livestock areas (Gutiérrez et al., 2018). An economically sustainable N-management is needed to

prevent NO3 leakage to groundwater (Rudolph et al., 2015; Gutiérrez et al., 2018; Li et al., 2021; Hamlin

et al., 2022). Mechanisms that would naturally remove nitrate from the soil, surface water reservoirs,

and aquifers, include denitrification and ammonification, both occurring under reduced conditions

(Tesoriero et al., 2021; Gutiérrez et al., 2022). Among the most promising methods for reducing loss of

nitrogen in irrigated arid areas are the synchronization of fertilization to coincide with rapid plant

growth, slowly released fertilizer, and nitrification inhibition. For example, sensor technology can

match the application of fertilizer to the nutritional needs of the plant during the different growth

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stages, and inhibitors of biological nitrification are commercially available (Norton and Ouyang,

2019; Drazic et al., 2020). However, these novel methods may take years to become an across-the-

board procedure used by farmers.

The Meoqui-Delicias aquifer in northern Mexico underlies an irrigated agricultural area within the

Chihuahuan Desert. Like other semiarid agricultural areas (Mukherjee and Singh, 2021; Re et al., 2021;

Liu et al., 2022), high levels of nitrate (> 10 mg/L NO3-N) in wells within this aquifer have been

reported (Espino et al., 2007; Barrera, 2008; Rascón, 2011). Despite this, groundwater quality studies

of this region are scarce, and a temporal analysis has not been reported for this region, nor the

distribution of N-species (NO3, N2O, NH4, Norg).

Although an upward trend in NO3 concentration is expected in this region due to its semiarid climate,

oxidizing conditions in the aquifer, and ongoing intense agriculture, this assumption needs to be

confirmed. The objectives of this study were to: (1) update the spatial distribution of NO3

concentrations for 2021 throughout the aquifer using data from 63 wells; (2) compare the 2021 NO3

values with those reported for these same wells in 2003 and 2006 to determine the variation with time

for each well and their possible source, and (3) plot the spatial distribution of NO3 variation to

identify the most affected and at-risk areas.

2. Materials and methods

2.1 Description of study area

The Meoqui-Delicias aquifer underlies an irrigated agricultural region known as Distrito de Riego

005 in northern Mexico (Fig. 1). The climate is semiarid, with an average annual precipitation of 284

mm. The Meoqui-Delicias aquifer occupies a surface area of 4,830 km2, has a maximum thickness of

about 500 m and an average thickness of 300 m. The aquifer is composed of alluvial fill, a material of

medium to high permeability, which is mainly recharged at alluvial fans present at the base of the

hills rising on its western and eastern parts. This is primarily an open aquifer, but under clay lenses

it operates as a confined aquifer (Villalobos-Gutiérrez, 2021).

Recharge in the aquifer occurs naturally and induced. In the first case, rainwater is collected directly

in the valley area and through underground horizontal flows coming from the surrounding foothill

areas. The induced recharge comes from the infiltration of surplus irrigation water, which contains

both groundwater and surface water (CONAGUA, 2020). This component in the groundwater

balance represents an important contribution to the vertical flow that allows for the introduction of

solutes to the unsaturated and saturated zones of the aquifer.

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Figure 1. Location of the Meoqui-Delicias aquifer (blue line), Irrigation District 005 (black line) and sampled

wells (red dots).

Figura 1. Localización del acuífero Meoqui-Delicias (línea azul), Distrito de Riego 005 (línea negra) y pozos

muestreados (puntos rojo).

The irrigated area grows summer and winter crops. Summer crops include mainly alfalfa, pecan nut

and jalapeño pepper, and to a lesser extent onions, sorghum, and cotton. Winter crops consist of

mainly forage grasses. Pecan nut has gained relevance in the region, for which its production has

increased significantly in recent years. This crop requires basic fertilizers or macroelements, such as

nitrogen (N), phosphorus (P) and potassium (K) to improve yield levels and product quality. Pecan

producing orchards are applied 80 to 100 kg of nitrogen per hectare; a large quantity considering that

nitrogen applications by gravity irrigation incur in losses between 30 and 45 % of the fertilizer

(INIFAP, 2013). Both pecan nut and jalapeño peppers receive large amounts of nitrogen fertilizers,

which are applied through irrigation. Some common choices of fertilizer include organic fertilizers

as urea and manure, but also a wide variety of mineral fertilizers such as potassium nitrate, potassium

phosphonitrate, ammonium nitrate and ammonium sulfate (INIFAP, 2015).

Alfalfa is a crop that, through the symbiotic relationship with atmospheric nitrogen-fixing bacteria

of the genus Rhizobium, produces its own nitrogen compounds. However, significant amounts of

organic fertilizers are applied in the area to improve soil quality and to retain moisture, as well as

ammonium phosphate to improve productivity (Lara and Jurado, 2014).

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This area is also an important dairy producer nation-wide, with more than 100 farms housing over

72,000 dairy cows (Rivas et al., 2008, 2018). The dairy farms are scattered within the irrigation district

DR-005, mainly in the municipalities of Delicias, Rosales, Meoqui and Saucillo, in the central part of

the aquifer. Among these, there are large dairy cattle farms in which advanced technology is applied

for the handling of livestock and dairy. However, small dairy farming operations are common as well

(Barrera, 2008). The wastes generated by dairy farms are used to irrigate nearby fields (Rivas-Lucero

et al., 2018), which allows us to assume that large amounts of solid and liquid waste with high

nitrogen content are spread over the fields, and that a part of it may leach into the groundwater

(Rivas-Lucero et al., 2018).

In addition, domestic sewage was mostly discharged into streams with limited to no treatment. In

this regard, a previous study by Espino et al. (2007) found that 34 % of 134 wells in this area contained

high NO3 concentration. A NO3-N source apportionment (manure, inorganic fertilizer, sewage) study

was conducted by Espino et al. (2011) using N-15 isotope analyses of 39 groundwater samples

corresponding to a small area of the study region. The results showed that 15 wells (38 % of total)

surpassed the drinking water limit of 10 mg/L NO3-N. Isotope analyses of 19 samples, including

urban and rural wells, found that mineral fertilizer was the source for NO3 in 11 % of the wells,

sewage, and manure in 52 % of the wells, and a combination of fertilizer and sewage and manure in

37 % of the wells. In rural areas, the source was a mixture of fertilizer residues, whereas in urban

areas, the source for 90 % of the analyzed wells was organic wastes.

Based on the results obtained by Barrera (2008) and Rascón (2011), groundwater is primarily of Na-

Ca-SO4-HCO3 type. A Piper diagram (Fig. 2) shows the water composition of representative wells

that are classified according to their location within the aquifer. Only 25 out of the 63 wells are

included in Fig. 2 to allow a better visualization.

Figure 2. Piper diagram showing major ion composition of groundwater according to regions within the

Meoqui-Delicias aquifer.

Figura 2. Diagrama de Piper que muestra la composición de iones mayores del agua subterránea por regiones

en el acuífero Meoqui-Delicias.

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2.2 Sampling and analysis

Groundwater from 63 wells were sampled from December 2020 to January 2021. The wells were

selected to match those previously studied by Espino et al. (2007), which had been originally selected

as to provide a good coverage of the area. Temperature, pH, and electrical conductivity were

determined in situ using a portable multi-parameter probe HANNA HI 9828 equipment. The TDS

values were obtained by direct conversion in this equipment based on the linear relationship between

both parameters. Samples were collected in 250 ml clean plastic bottles and kept cool during their

transport to the laboratory. Once in the laboratory, NO3-N concentrations were measured with a

Hach DR/890 colorimeter using the cadmium reduction method. In this method, NO3 is reduced to

nitrite (NO2) and measured together as NO3+NO2 (Villalobos-Gutiérrez, 2021). Due to the oxidizing

conditions in this and adjacent aquifers (Mahlknecht et al., 2008; Reyes-Gómez et al., 2015;

CONAGUA, 2022), the amount of NO2 is about two orders of magnitude smaller than NO3, for which

(NO3+NO2) can be approximated to NO3. Chloride concentrations were determined by tritation using

the argentometic method (Secretaría de Economía, 2001). Blanks of three-distillated water were used

for equipment calibration and at least one out of every 10 samples were run in duplicate.

2.3 Data processing

The locations of the wells were plotted using ArcMap and nitrate isoconcentration lines were

constructed at 2 mg/L NO3-N interval for the 2021 data to show the spatial distribution of NO3-N

concentrations. To observe the evolution of this parameter over time, results obtained in the sampling

campaign carried out in January 2021 (2021 data) were compared with values reported by the

Comisión Nacional del Agua (CONAGUA) in 2003 and with samples collected in 2006 reported by

Barrera (2008) and Rascón (2011) for the same wells.

For temporal variations, the total number of samples was reduced from 63 to 60 after removing wells

missing a measurement. Each well was classified into one of five possible groups based on the slope

of the line obtained in the regression analysis applied to the variation in nitrate concentration with

respect to time: increase, minor increase, no change, minor decrease, and decrease. The small number

of available measurements (3) precluded a formal calculation of a trend, which requires a minimum

of eight measurements (Ducci et al., 2020). Therefore, a simplification was devised to provide an

estimation rather than a calculation of a concentration trend based on the regression coefficient of the

best-fitting line of NO3-N versus time graphs. The slope of its regression line was used as an indicator

value of the trend in the variation of the concentration.

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2.4 Nitrate Pollution Index

The Nitrate Pollution Index (NPI) for 2021 data was calculated according to the formula below

(Obeidat et al., 2012):

NPI = 𝐶 − 𝐻𝐴𝑉

𝐻𝐴𝑉

(1)

where C is the analytical concentration of nitrate in the sample and HAV is the threshold value of

anthropogenic source (human affected value) taken as 4.51 mg/L NO3-N (20 mg/L NO3).

3. Results and discussion

The water quality results of the 2021 sampling campaign are listed in Table 1. Wells are labeled

with a letter corresponding to the nearest town (D for Delicias, J for Julimes, LC for La Cruz, M for

Meoqui, R for Rosales and S for Saucillo) followed by a number.

Nitrate concentrations varied between 0.7 and 23.2 mg/L NO3-N and had an average of 7.5 mg/L NO3-

N. Most of the wells (62 %) had a modest nitrate content, 22 % of the wells surpassed the WHO

guideline of 10 mg/L NO3-N, and 16 % had low values (< 3 mg/L NO3-N). These results agree with

the obtained NPI results, which show that the sampled wells cover all levels of pollution and overall

has a moderate NO3 pollution, with 33.3 % of the wells having moderate to very significant NO3-N

concentrations. The NPI results are listed in Table 2. Another important observation from data in

Table 1 is the NO3-N /Cl ratio < 1 in all cases, which according to Linhoff (2022) relates nitrate with

an anthropogenic origin derived from human and animal manure.

Table 1. Water quality data of wells in the 2021 sampling campaign

Tabla 1. Datos de calidad del agua de los pozos en la campaña de muestreo 2021

Well

T °C

TDS

mg/L

NO3-N

mg/L

NO3-

N/Cl

Well

T °C

TDS

mg/L

NO3-N

mg/L

NO3-

N/Cl

D119

18.2

7.1

1128

11.5

0.34

M23

18.7

8.0

690

17.0

0.27

D126

21.3

7.3

775

11.8

0.27

M24

21.9

7.4

539

11.4

0.15

D127

23.5

7.4

1333

8.0

221

0.05

M25

22.0

7.3

604

10.4

0.32

D129

26.1

6.4

784

23.2

0.23

M26

27.2

7.5

355

11.2

n.d.

D130

18.2

7.3

2059

10.8

363

0.03

M27

27.7

7.1

675

15.8

n.d.

D133

24.0

6.7

1307

20.1

0.13

M39

10.8

7.9

383

4.6

0.89

D134

24.7

7.5

862

9.8

n.d.

M40

21.8

7.0

398

4.9

0.21

D136

25.7

7.2

863

7.0

n.d.

M41

17.5

7.4

434

7.0

0.22

D137

20.0

7.3

519

5.9

0.93

M42

22.3

7.5

618

9.0

0.10

D138

21.4

6.0

1057

15.0

0.08

M43

24.3

7.4

727

5.1

0.12

D139

25.0

7.7

520

4.4

0.53

22.4

7.4

587

5.4

0.20

18.3

8.3

1056

0.7

149

0.03

17.5

7.5

429

4.2

0.38

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J10

22.9

7.1

1587

1.0

273

0.03

24.3

8.1

490

2.0

0.05

J11

23.1

7.5

1031

1.4

118

0.02

R30

15.9

8.2

418

1.4

0.51

J12

22.3

7.1

1429

2.6

108

0.04

R35

23.1

6.9

951

7.9

0.04

J13

22.2

7.1

1429

2.7

101

0.06

R38

12.8

7.6

330

2.5

0.22

J14

19.0

7.3

1221

4.4

0.07

S44

19.9

7.6

583

5.2

0.26

J15

23.5

7.0

1193

2.4

106

0.09

S45

21.1

7.8

575

7.8

0.30

J16

19.8

7.4

890

2.2

0.03

S46

21.2

7.0

817

17.8

n.d.

J17

13.7

7.5

720

1.8

0.04

S47

22.4

7.0

735

7.3

n.d.

J18

25.5

7.2

840

5.2

0.05

S50

23.0

7.5

581

13.4

n.d.

LC63

18.6

7.3

762

6.6

101

0.08

S52

23.0

7.6

755

19.8

n.d.

LC65

21.9

7.8

914

3.5

n.d.

S54

21.0

7.3

786

7.2

n.d.

LC67

21.6

7.2

739

5.2

0.28

S56

24.6

6.7

834

9.0

n.d.

LC69

25.1

7.4

884

6.2

0.18

S562

27.0

7.2

843

10.9

n.d.

20.9

7.3

401

4.6

0.17

S57

24.9

7.6

456

4.5

n.d.

24.5

8.3

469

3.6

0.59

S58

15.4

7.2

1020

7.2

0.29

18.7

7.9

953

11.9

113

0.11

S59

22.4

7.6

444

5.9

0.44

M19

14.1

8.0

701

8.9

n.d.

S60

22.4

7.3

454

7.3

0.27

M21

22.1

7.2

739

4.2

0.27

S62

23.4

7.4

793

6.6

0.16

M22

25.2

7.5

842

3.4

0.15

S98

22.2

7.2

668

9.6

n.d.

M22-2

13.5

7.2

1000

2.5

0.33

n.d. = not detected

Table 2. Nitrate pollution index (NPI) classification of groundwater (Obeidat et al., 2012) for 2021 samples

Tabla 2. Clasificación del agua subterránea de acuerdo con el Índice de Contaminación por Nitrato (NPI)

(Obeidat et al., 2012) para las muestras 2021

NPI value

Degree of

pollution

No. wells

% wells

< 0

Clean

27.0

0 - 1

Light

39.7

1 - 2

Moderate

20.6

2 - 3

Significant

7.9

> 3

Very significant

4.8

The nitrate concentration values reported for these wells in 2003 and 2006 were added to the 2021

data in orden to compare the changes of NO3-N concentrations with time. The data are listed in Table

3 and their spatial distribution in Figure 3, with shaded areas corresponding to high NO3-N

concentrations. After visually comparing the three maps (Fig. 3), one can see that the high NO3-N

concentrations areas shift with time but there are some areas with persistent high concentrations.

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Table 3 Nitrate values (in mg/L NO3-N) of studied wells in 2003, 2006 and 2021

Tabla 3 Valores de nitrato (en mg NO3-N/L) de los pozos estudiados en 2003, 2006 y 2021

Except for two locations where the pH values were 6.0 and 6.4 (Table 1), the pH values were within

the drinking water limit of 6.5-8.5 set by the World Health Organization, favoring the alkaline

condition. The pH values below 7.0 likely result from the infiltration of leachate from dairy farm

wastes, which have an acidic pH (Rivas-Lucero et al., 2018). In contrast, TDS values seemed to

associate with irrigation that extends to about 30 km radius, in localities water enriched with salts

Well

Depth

NO3-N

2003

NO3-N

2006

NO3-N

2021

Well

Depth

NO3-N

2003

NO3-N

2006

NO3-N

2021

D119

17.2

12.5

11.5

M23

13.9

9.1

17.0

D126

10.7

9.4

11.8

M24

5.4

7.4

11.4

D127

10.6

5.9

8.0

M25

8.8

10.1

10.4

D129

14.9

12.1

23.2

M26

150

10.1

9.5

11.2

D130

320

12.1

12.5

10.8

M27

150

11.8

15.5

15.8

D133

11.4

14.8

20.1

M39

6.7

5.4

4.6

D134

5.3

8.3

9.8

M40

150

4.0

4.8

4.9

D136

200

10.2

9.6

7.0

M41

5.8

6.4

7.0

D137

7.1

10.0

5.9

M42

4.8

3.4

9.0

D138

6.8

8.9

15.0

M43

5.3

6.3

5.1

D139

17.7

20.0

4.4

152

11.9

10.7

5.4

4.2

3.9

0.7

11.9

6.3

4.2

J10

8.5

2.5

1.0

2.5

4.6

2.0

J11

1.9

1.4

R30

9.6

1.5

1.4

J12

4.2

5.1

2.6

R35

3.2

7.9

J13

5.8

4.2

2.7

R38

2.5

1.4

2.5

J14

6.2

4.9

4.4

S44

10.8

4.5

5.2

J15

9.5

4.0

2.4

S45

7.0

12.3

7.8

J16

1.3

2.2

S46

34.3

25.3

17.8

J17

3.3

4.0

1.8

S47

11.8

12.6

7.3

J18

3.1

1.9

5.2

S50

8.9

8.5

13.4

LC63

8.2

9.9

6.6

S52

7.0

5.9

19.8

LC67

9.2

4.2

5.2

S54

6.5

11.3

7.2

LC69

9.0

6.4

6.2

S56

185

8.1

9.0

5.1

6.6

4.6

S57

180

13.0

5.6

4.5

19.0

13.4

3.6

S58

250

13.6

7.2

181

12.3

14.8

11.9

S59

13.0

12.0

5.9

M19

8.3

5.4

8.9

S60

7.4

6.9

7.3

M21

7.8

6.4

4.2

S62

6.5

7.0

6.6

M22

12.5

3.6

3.4

S98

137

4.5

9.6

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after evaporation or from being in contact with salt-rich soils. TDS values exceeded the WHO norm

(1,000 mg/L) in twelve wells spread over two main areas, one at the center of the aquifer (Meoqui and

Delicias municipalities) and the other in its discharge area northeast corner of the aquifer, where the

Rio Conchos receives irrigation return flows (Julimes municipality). In contrast, the lowest TDS

values, around 400 mg/L, were observed in wells located near the recharge areas in the western part

of the aquifer.

Figure 3. Evolution of NO3-N concentrations in 2003 to 2021. Zones >10 mg/L NO3-N shaded in pink.

Figura 3. Evolución de las concentraciones de NO3-N en 2003 a 2021. Las zonas con NO3-N >10 mg/L están

sombreadas en rosa.

The moderate NO3-N values for most samples and a ratio of NO3-N/Cl < 1 in all samples (Table 1)

support an anthropogenic origin of nitrate, and thus a negligible presence of natural subsoil NO3-N

deposits (Linhoff, 2022). The spatial distribution of 2021 NO3-N concentrations in Fig. 3 shows the

highest concentrations in the eastern part of Delicias, spreading over an area that belong to the

municipalities of Delicias, Meoqui and Saucillo. Important areas of cultivation and large dairy farms

are also located there. In addition, sources of pollution also include inadequately treated domestic

waste from rural communities, such as septic tanks and other waste treatment systems.

At this point in time (2021 data), having one third of the wells in the categories of moderately to

highly polluted, it seems crucial to find out if this is a relatively stable behavior or if the aquifer is

borderline to worsening. Thus, NO3-N data reported for 2003 and 2006 were added (Table 3) and

plotted in maps displayed next to each other for comparison purposes (Fig. 3). To ease the

comparison, areas where NO3-N >10 mg/L were shaded in pink. Comparison among these maps

shows that the high NO3-N concentrations vary with time. One shaded area (NO3-N >10 mg/L)

located in the center of the aquifer persists through the years, extending south towards Saucillo in

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2021, and encompassing wells from Delicias, and Saucillo municipalities. Interestingly, two areas of

high NO3-N concentrations in the north and western part of the aquifer decreased in concentration

in 2021.

A quantitative approach was then necessary to denote their variation with time. A first glance to the

data in Table 3 indicates little variation (1 to 10 mg NO3-N/L) in NO3-N concentrations for most wells

between 2003 and 2021. The linear regression analysis of NO3-N concentrations for the years 2003,

2006 and 2021 shown in Table 3 was used to determine the trend in variation over time for each well,

based on the slope of their regression line (R). A visual inspection of the range of R and the slope

indicated that some wells had an imperceptible change with time while others had a well-delineated

change (either increase or decrease). Once those two were separated, adding an intermediate range

was necessary to include all data. The selected five categories and the distribution of wells into each

of these categories are listed in Table 4.

To help distinguish which wells increased their NO3 content, they were separated by code letter

(municipality). Once this was done, D-wells (Delicias) were more abundant in the increase category

and relatively absent in the decrease category, a behavior that was closely followed by the M

(Meoqui) wells. In total, an increase behavior was observed in 25 % of the wells. On the other hand,

45 % of the wells showed a decrease or minimum decrease, among which wells from Julimes and

some of Saucillo stood out. Regarding the increased number of wells whose nitrate concentration was

lower in 2021, it is important to point out that these samples were collected during the winter, which

is a time of low agricultural activity, unlike the samples of previous years collected in summer. On

the other hand, the climatic factor does not affect milk production activity, which justifies the

observed increase in nitrates in wells located adjacent to the largest dairy farms.

The overall results show that many of the D-, M- and S- wells (located in the Delicias, Meoqui, and

Saucillo municipalities, respectively) contain high concentration of NO3-N and identifies this area is

a vulnerable part of the aquifer. The nine most affected wells were M23 (Loreto), M24 (Puentes), M27

(Nuevo Loreto), D129 (La Merced), D133 (Virginias), D138 (Nicolas Bravo), S46 (Santa Rosa), S50

(Altamirano) and S52 (Madero). Although not yet affected in the same proportion, other

municipalities that have the same alluvial fill (e.g., Julimes, La Cruz, and Rosales) may become

affected in the future if the amount of N-fertilizers, manure and other animal and human wastes were

to increase.

Besides the small variation in NO3-N concentrations overall (Table 3), one can note that there are

more wells in the decrease category than in the increase category (Table 4). Among the possible

reasons that could explain the decrease observed in 2021 with respect to 2003 and 2006 are: 1) the

proximity of recovered wells to the natural recharge zones of the aquifer after a wet rainy season in

late 2020; 2) more effective application of nitrogen fertilizers by farmers in 2020; 3) decrease in nitrate

concentration from the unsaturated zone due to denitrification; and 4) a better containment of animal

wastes in farms near the recovered wells. However, more information would be necessary to validate

each of the above points.

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Table 4. Slope of regression line and number of data falling into each of the five assigned categories

Tabla 4. Pendiente de línea de regresión y número de datos que caen dentro de cada una de las cinco

categorías asignadas

Category

Slope

Number

of Wells

Wells

increase

> 0.290

D129, D133, D138, M23, M24, M42, R35, S50,

S52, S98

minor increase

0.090 to 0.290

D126, D134, J18, M19, M27

no change

-0.089 to 0.089

D127, D130, J11, J14, J16, M1, M6, M25, M26,

M40, M41, M43, R7, R38, S54, S56, S60, S62

minor decrease

-0.090 to -0.290

D119, D136, D137, J9, J12, J13, J17, LC63,

LC67, LC69, M21, M39, S44, S45

decrease

< -0.290

D139, J10, J15, M5, M22, R2, R3, R30, S46,

S47, S57, S58, S59

The spatial distribution of the variation categories is shown in Fig. 4 for a visualization of points in

the study area that experienced the most variation (red dots for increase, blue dots for decrease) and

for those wells where no change was observed (light green). The color-coded wells in Fig. 4 scatter

throughout the area instead of grouping in a zone where a certain tendency (increase or decrease)

prevails. However, we can observe that the wells with tendency to increase (red dots for high increase

and orange dots for low increase) scatter within the central part of the aquifer. These wells plot the

central part of the aquifer in the same region where, as we can observe in Fig. 3, high NO3-N

concentrations are found, coinciding with the location of large dairy farms (Fig. 4). On the other hand,

the areas with no change (green dots) or decrease in nitrate concentration (blue dots) correspond to

recharge zones at north and south of the aquifer, identified by their higher altitude, i.e., the mountain

ranges that limit the aquifer.

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Figure 4. Spatial distribution of the variation of NO3-N with time according to the five variation intervals: high

increase (red), minor increase (orange), no change (light green), minor decrease (light blue), high decrease (dark

blue). Regional groundwater flow (blue arrow) and location of largest dairy farms shown for reference purposes.

Figura 4. Distribución espacial de la variación de NO3-N con el tiempo de acuerdo con los cinco intervalos de

variación: incremento alto (rojo), incremento bajo (naranja), sin cambio (verde claro), decremento menor (azul

claro), decremento alto (azul oscuro). Flujo regional del agua subterránea (flecha azul) y localización de grandes

establos mostrados con propósito de referencia.

A further attempt to explain the spatial distribution of NO3-N concentrations and their variation with

time was to consider the well depth. Well depth information was found only for 18 of the 63 wells

and was added to Table 3. The depths vary between 15 and 320 m. One should recall from Section 2

that the average depth of the aquifer is 300 m but can be up to 500 m in places. Especially troubling

was to find concentrations above 10 mg/L NO3-N in 50 % of the deep wells (150-320 m deep). This

finding points to nitrate contamination in parts of the aquifer generally considered to be free of

anthropogenic contamination. Again, the disposal of animal waste was considered a likely source of

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contamination because of the high content of nitrogen in animal waste spills and the high NO3-N

concentration locations roughly coinciding with the areas where large dairy farms operate. It is

possible that oxidation of dairy farm waste transforms the nitrogen compounds to nitrate (a soluble

compound), which is readily transported by water into deeper parts of the aquifer.

Although NO3-N contamination is not yet severe in the study area, actions to prevent contamination

leakage to groundwater are advisable, especially in the areas identified as vulnerable. Recommended

actions include improving the waste management of manure and the application of only the needed

amount N-fertilizer to reduce N-losses (Rivas-Lucero et al., 2008; Millar et al., 2018). Also, the

successful implementation of any best management practices requires attention to social aspects and

a clear and sensible communication between stakeholders, e.g., farmers, city officials, and water

managers (McCullogh and Matson, 2016), as farmers often resist the adoption of new procedures

until they are convinced of their effectiveness. This step alone may take several years (McCullogh

and Matson, 2016).

Among the preventive methods known to reduce leakage of nitrate to the aquifer, the planting of

cover crops stands out. Planting a cover crop has been implemented in the Meoqui-Delicias region

for decades, although not by all farmers and, in the past few years, water-efficient methods such as

drip-irrigation have started to become a new norm. Other preventive methods that are promising but

have not been implemented in the region include no-tilling and the use of automatic sensors for

fertilizer and water application (Gutiérrez et al., 2021). The use of sensors has been reported as an

effective and sustainable method but can be costly (Norton and Ouyang, 2019; Drazic et al., 2020).

Examples of corrective methods to reduce NO3 concentrations include the chemical treatment of

contaminated water by adsorption to a variety of materials or the use of bio-barrier substrates for

nitrate removal by denitrification (Özkaraova et al., 2022).

4. Conclusions

Groundwater NO3 concentrations varied widely from 0.7 to 23.2 mg/L NO3-N throughout

the study area. The distribution of nitrate content and the ratio NO3-N/Cl<1 suggests an

anthropogenic origin of this contaminant related to human and animal wastes, which should be

verified in future research. The spatial distribution of NO3-N concentrations, according to the

reviewed literature and information on the area, indicates a possible association with waste from

dairy farms, leaking of domestic wastewater, and/or excess of applied N-fertilizer. Specific studies

are proposed to corroborate this assumption. According to NPI, 2/3 of the wells are slightly- to non-

polluted, and 1/3 are moderately to very polluted. The distribution of the most affected wells was

relatively scattered, which suggests the discharge of large amounts of waste at a few specific points.

After comparing their variation in concentration from 2003, 2006 and 2021, a small increase in

concentration was observed in 17% of the wells, a decrease in 22% of the wells and minor to no

change in 61% of the wells. The central part of the aquifer had the highest NO3-N concentrations and

increasing trends; however, wells with less nitrate were also present. Despite relying on only three

years of data, the approach followed here successfully identified the affected wells, although the

trends need to be confirmed using more years’ data. Some deep wells surpassing the 10 mg/L NO3-

N concentration guideline was a concerning find. The implementation of preventive measures to

curb pollution, such as an efficient application of N-fertilizer, water-efficient irrigation, and better

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practices in disposing of dairy farm wastes, are needed to move towards sustainably managing this

aquifer.

Author contributions

Conceptualization: M.S.E.V. and M.G.; methodology: M.S.E.V., H.S.H. and A.P.M.; software:

N.V.G., H.S.H. and A.P.M.; validation: M.G., A.P.M. and H.S.H..; formal analysis: N.V.G. and M.G.;

investigation: M.S.E.V. and N.V.G.; resources: M.S.E.V. and M.G..; data curation: N.V.G..; writing-

original draft preparation, M.S.E.V. and N.V.G.; wrinting-review and editing, M.G.; visualization:

M.G..; supervision: M.S.E.V..; Project administration: M.G..; funding acquisition: M.S.E.V. All

authors have read and agreed to the published version of the manuscript.

Conflict of Interest

Authors have no conflict of interests to declare that are relevant to the content of this article.

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