Model structure

This section includes: description of the study area, sub-models comprising the SIWM model, and a simplified graphical representation of the model structure.

4.3.1. Study area

The study area encompasses a rectangular area of about 100 km x 45 km that includes the MAM defined by the Yucatan government along with peripheral municipalities of the MAM (Figure 23). The geographic orientation of this area is based on the groundwater flow direction (southeast to northwest), to consider pollutant transport along with groundwater flow.

This area is divided into 4 equally-sized sections: A, B, C and D from southeast to northwest direction, referred to as “aquifer sections”. Each section has its own characteristic for example in terms of population and wastewater load (both are highest in C that includes majority of Merida City and lowest in A, a predominantly rural area).

4.3.2. Sub-model structure

The model includes 10 sub-models, representing the water cycle (aquifer sub-model) and human influence (9 sub-models). Aquifer sub-model was conceived as the environmental factor for the conceptual model of this research (Figure 20). It comprises water quality data as input and output from the MAM case study.

Table 21 Sub-model structure of the present research

Factor Group Sub-model Coding name in the model

Water cycle Aquifer Aquifer AQUIFER

Human

Population Population POP

Urban Domestic Urban DU

Industry IND

Institution INS

Public Urban PU

Rural Domestic Rural DR

Agriculture AGR

Aquaculture AQU

Livestock LIV

The model was developed with data from the 4 aquifer sections of the MAM case study (Figure 23), which was delimited considering the site-specific groundwater pollution documented and data availability from local research. The human influence comprises 9 sub-models. The population sub-model, which affects all the other 8 sub-models that represent the anthropogenic activities and are classified into two groups: “urban” and

Figure 23 Rectangular study area of the model with the four aquifer sections A, B, C and D.

Intermediary

Pre -data Post -data

Figure 24 Data flow of the Sustainable Integrated Water Management (SIWM) model

“rural”, as shown in Table 21 together with their coding name used within the model environment.

Figure 24 shows the 3 stages of data flow from input data, data processes and calculations, and final outputs. Once the most significant health risks associated with drinking water are identified, by quantifying pollutants concentration in the aquifer, potential public health interventions are modelled. Quantitative Microbial Risk Assessment (QMRA) was performed in order to estimate the burden of diseases for specific pollutants. Finally, cost-benefit analyses for the public health interventions are evaluated to identify interventions that could most effectively tackle the highest public health risks related to specific pollutants.

In order to explain the relevance of each of ten activities (sub-models in the SIWM model), a brief description is presented below. These impacts qualitative and quantitative to the water cycle through the pollutants loads estimated as wastewater discharged. Pollutants load values were estimated based on the total production or number of people (i.e. workers, students) per specific activity, which were obtained from statistical databases of the MAM case study.

1. AQUIFER. This is segmented in 4 sections (A, B, C, and D), taking into consideration the very different water consumption and wastewater generation within each section. It serves to integrate the 8 human activities-based sub-models in order to represent a holistic scenario of the water cycle for the MAM case study, and the scenario analysis for all interventions.

2. POP. This sub-model was built as a dynamic element linked to the growth rate for the 8 activities, which are depending on population growth.

3. AGR. It comprises three main agricultural groups: vegetables, fruits and maize.

Vegetable group includes tomatoes, cabbage, onion, and pepper. Fruits group

Data input :

includes watermelon, pineapple, banana, and citrus. Maize was treated as single group due to its significantly high production and water requirements.

4. AQU. It includes two main aquaculture groups: shrimps and fish. Due to the significant production, shrimps are considered a separate group, whereas the rest of the aquaculture production is grouped as “fish”, including mainly tilapia and ornamental fishes.

5. LIV. It includes four main livestock activities: porcine, ovine, poultry and bovine.

Poultry comprises: chicken, turkey and duck production.

6. DR. This includes rural households classified according to the type of wastewater treatment currently operating: HH1. Households connected to septic tanks (HH-ST), HH2. Households connected to improved septic tanks (HH-iST), as an intervention; HH3. Households connected to wastewater treatment plant (HH-WWTP), and HH4. Households connected to improved wastewater treatment plants (HH-iWWTP) – as an intervention.

7. DU. This includes the same groups as in DR.

8. PU. This includes two groups: services and trade. Trade comprises shopping centers, city market, and small shops. Service includes all public spaces such as parks, museums, stadiums, etc. in the urban area.

9. INS. This comprises hospitals, hotels, and offices and schools. Offices and schools are grouped together because of the similarity on water consumption and type of pollutants discharged.

10. IND. This includes two main groups: manufacture and construction.

Manufacture comprises food, plastics, wood, metals, textile, mechanic, electronics, and other manufactured products. Construction includes all registered contractors and activities related to the construction sector, including sites of materials extraction.