prediction of flow in a karstic aquifer, case yumagual ... · the study area is located in the...

E-proceedings of the 38th IAHR World CongressSeptember 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-0124

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PREDICTION OF FLOW IN A KARSTIC AQUIFER, CASE YUMAGUAL, CAJAMARCA, PERU

Edwin Pino-Vargas (1), Jesus Mejía-Rueda (2), Jesus Mejía-Marcacuzo (3), Andre Steenken (4), Luis Alfaro-Ravello (5) & Cesar Avendaño-Jihuallanga (6)

(1,4,5,6) Universidad Nacional Jorge Basadre Grohmann, Tacna, Perú, [email protected], [email protected], [email protected], [email protected]

(2) Schlumberger del Peru S.A., Aguas Subterráneas, Lima, Peru, [email protected]

(3) Universidad Nacional Agraria La Molina, Lima, Perú

[email protected]

ABSTRACT

The study describes the variables to establish the conceptual hydrogeological model and define the hydrogeological units, the recharge zones, the discharge zones, the direction of flow, the piezometric behavior, among others. The hydrogeological numerical model was developed in steady state (natural conditions) and transitory regime. The error percentage in the modeling is 6%, which is considered acceptable when dealing with such heterogeneous media. The permeability data were obtained from the permeability and pumping tests carried out in the area, obtaining the following ranges: Agglomerates of Volcanic Tuffs of 5.00•10-9 to 9.40•10-6 m/s, Diorites of 2.40•10-10 to 2.36•10-6 m/s, fault breccia of 8.10•10-10 to 2.02•10-6 m/s, Limestones from 3.50•10-10 to 1.02•10-5 m/s, Dioritic porphyries from 1.60•10-10 to 3.36•10-5 m/s, Rhyolites from 3.40•10-9 to 1.20•10-6 m/s, Rhyolitic breccias of 5.00•10-9 to 5.70•10-7 m/s, Rhyolitic tuffs of 3.10•10-9 to 1.76•10-6 m/s, Sands and gravels of 2.61•10- 5 to 9.83•10-5 m/s and Soils: 1.00•10-6 m/s. With the numerical model in transitory regime, a simulation was carried out to establish the total drainage flow, which ensures that the water table is 10 m below the ground level, according to the mining plan of pit 01. The flow was estimated of drainage of the pit and the drainage of the pit was simulated under three scenarios based on the pumping flow rates of the existing pumping wells.

Keywords: Karstic aquifers; Yumangal aquifer, numerical simulation

1 INTRODUCTION The study area is located in the region of Cajamarca, province of Hualgayoc. In this area, mining operations

are located for the exploitation of an open-pit copper deposit. The predominant rock in the area is of calcareous origin, the same that presents karstification and by these calcareous units the groundwater is mobilized. Currently, numerical modeling is a tool that has been applied increasingly in the field of hydrogeology, its purpose is to predict the hydraulic behavior of groundwater, in different scenarios. In the last decade, mining in Peru has been growing and with it the demand to have tools that simulate the behavior of groundwater, in any scenario, especially karst media, which occur in many regions of the country. In general, karst aquifers are considered to have properties that are clearly different from other mother-rock aquifers. We have found in a bibliographic search five definitions that have been proposed to differentiate karst aquifers from non-karstic aquifers. The five definitions are based on the presence of channel networks, hydraulic conductivities > 10-6 m/s, karstic systems, channels with turbulent flow and caves. The percentage of unconfined carbonated aquifers that would be classified as “karst” is between < 1 and > 50% (Worthington et al., 2017).

In the field of karstic hydrogeology, different proposals have also been made for cataloguing springs; the main variables used were discharge, geological, tectonic and morphological conditions, or the water origin and provenance (Llopis Lladó, 1970). The nature of the flow in these complex systems, remains an important research challenge in karst hydrology (Gabrovšek et al., 2018). Based on data from groundwater geochemistry, stratigraphy, surface and tectonic features, it is possible to identify ancient karst systems (Perry et al., 2010). The hydrological and chemical differences are caused in part by the lower altitude of the Yucatan plain. More importantly, however, these differences are due to the lack of a superior confinement bed in the Yucatan that is hydrologically equivalent to Florida's Hawthorn Formation. The Hawthorn cover prevents recharge and confines artesian water except where it is drilled by sinkholes, but sands and other unconsolidated sediments fill

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sinkholes and cavities and impede circulation. In the Yucatan, the permeability of the entire section is so enormous that rainfall infiltrates immediately into the phreatic surface and then moves laterally towards the discharge areas along the coasts (Back & Hanshaw, 1970).

The southwest of China is a typical karstic zone (it covers approximately 620 000 km2) with a population of approximately 100 million. Karst groundwater is recognized as an essential water resource for human consumption and agriculture. The hydrogeochemical evolution of groundwater is typically affected by natural factors, including the composition of rainwater, the geological structure and mineralogy of the aquifer, and the water-rock interaction along the flow paths. In addition to natural processes, anthropogenic activities can also strongly influence the hydrogeochemical characteristics of groundwater. In light of the sustainable management of groundwater resources and scientifically challenging in complex karst scenarios, understanding the dominant processes governing groundwater, hydrogeochemical evolution is critically necessary (Yuan et al., 2017). The APLIS method (Andreo et al., 2004) is based on a GIS analysis where the main variables that influenced the recharge are taken into account, with special emphasis on the carbonated aquifers. The methodology is applied to carbonate aquifers of the tropical mountains of Viñales National Park. As the method was developed in the Mediterranean conditions, some modifications were necessary. The results of the sensitivity analysis of the map removing show that the relative influence in the final recharge map is L> I> S> P> A (Lithology (L), Infiltration (I), Soil (S), Slope (P), Altitude (A)). The single parameter sensitivity analysis shows some deviations between the empirical and the real weighting (Farfan et al., 2010). The proposed method has been successfully applied in eight carbonate aquifers of the Betic Cordillera, with different climatic, geological, geomorphological, topographic, soil and hydrogeological characteristics. In this sense, it is considered that the APLIS method may be especially applicable to carbonate aquifers of alpine peri-mediterranean mountain ranges (Andreo et al., 2004).

The epikarst, a zone of increased weathering near the land surface, determines the distribution of recharge to a karst aquifer in both space and time. It links climatic and nearsurface geological conditions with the karstification of a limestone aquifer, defining both the hydraulic and the chemical boundary conditions for the development of the karst system (Bauer et al., 2005). The hydrological balance of the karstic aquifer of the Duero sedimentary basin, in central western Spain, was established with the obtained results, having been introduced into a simulation tool in order to know the temporal evolution of water resources throughout the hydrological year. For this purpose, it was necessary to use geostatistical techniques and numerical modelling (Visual Modflow program). It is a sedimentary karst unconfined aquifer, and its balance is determined by rainfall and drainage (Sanz-Lobón et al., 2015). Understanding the functioning of karst aquifers, under management and protection, poses unique challenges. Karstic aquifers are characterized by the fact that the flow of groundwater is through conduits (tertiary porosity), and/or layers with interconnected pores (secondary porosity) and intergranular porosity (primary porosity or matrix) (Kuniansky, 2016).

The APLIS is very important tool in karst aquifer systems for the establishment of operational drainage decisions and their environmental effects. From this comes the following question: how reliable and precise can be the results obtained from a hydrogeological numerical model? First, it is necessary to specify that numerical modelling will depend very much on the conceptual hydrogeological model and this will depend on the quantity and quality of information available on the aquifer units. The aquifer's hydraulic information, groundwater chemistry, hydrology, geology, geological structures, civil structures, among others, are variables that the hydrogeologist must consider to understand the functioning of the aquifer and formulate the reliable hydrogeological model.

2 MATERIAL AND METHODS

The study area is located in the province of Hualgayoc, in the district of Cajamarca, in northwestern Peru, 2 km from the town of Hualgayoc and 50 km north of Cajamarca (Fig. 1). Topographically, it is located in a mountainous area with elevations between 3,500 and 4,000 meters above sea level. Hydrographically the area is located on the western slope of the Peruvian western mountain range. The study area is located between the Tingo River to the north and Hualgayoc to the south and the Quebrada Mesa de Plata to the east. In the study area, a mining project is operating for the copper extraction, by means of the open-pit modality.

Geological and structural characteristics Based on the existing geological information and a mapping campaign, a three-dimensional geological block

was prepared for the study area. For this a litho-stratigraphic column, geological mapping of the study area including map of outcrops, main structures and details on geometry of the strata (dips) is used, including interpretive geological profiles. In the area of study there are outcrops of sedimentary rocks from the lower, middle Cretaceous, Quaternary and intrusive Paleogene and Neogene rocks present in the form of stocks, sills, dikes and porphyries that intrude on Cretaceous formations. Structurally, there is great complexity associated


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with the intense faulting that affects the different formations, although two main fault directions can be differentiated: NO-SE and NE-SO.

Figure 1 | Location of the study area.

Hydrogeological characteristics A total of 271 springs were identified with average flow rates between 0 and 47.09 l/s. These springs are

linked to the discharge of permeable formations by cracking/karstification, as they present a markedly seasonal discharge regime, in response to a system of high diffusivity (low hydrodynamic inertia). Once the database of springs had been structured, an analysis of the nature of these springs was carried out. The purpose of the analysis is to establish and differentiate the natural discharge points of the underground flows (springs) and the drainage points associated with sub-surface flows, and therefore not linked to the underground system. To establish this classification, a contrast was made with the geological (3D) and piezometric (isopieza map) data. 170 drainage points associated with sub-surface flows with average flow rates between 0 and 38.72 l/s were identified and 95 springs, with flow rates between 1.00 and 53.00 l/s (10 springs with flows above 10 l/s and 6 springs with flows above 25 l/s). In the study area there are 180 piezometers (generating 417,510 data on the piezometric level), taken between October 2007 and June 2012. The data were bunched together into seven groups, according to their geographical location within the mining environment (pit, north pit, plant, South, intakes, tailings dam and North). Of the 180 piezometers, 74 piezometers were selected with a large number of piezometric records over time.

Conceptual model It is a heterogeneous complex and anisotropic medium, consisting of 4 hydrogeological units, depending

on their behavior and hydrogeological parameters. These units are: (a) Hydrogeological unit 1: permeable rocks by cracking and karstification. (b) Hydrogeological unit 2: permeable materials by intergranular porosity. (c) Hydrogeological unit 3: rocks with low permeability due to fracturing. (d) Hydrogeological unit 4: low permeability rocks.

In general, these units (intrusive rocks and limestones) have low primary permeability due to their genesis, but due to fracturing and karstification processes, a high secondary permeability was developed in determined favourable areas. Permeable formations are represented by Cretaceous limestone with karstificación (Fms. Yumagual and Mujarrún) and the surficial permeable of Quaternary formations (colluviums and alluvial), which


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make up local aquifers of limited importance. The geological structures (fractures) of the zone, have a great importance because they allow the hydraulic connectivity between the different hydrogeological units reported. Therefore, for practical purposes, these hydrogeological units were considered to form a single hydrogeological system for the study environment.

The intrusive rocks of the pit (included in UH 3: rocks with low permeability due to low fracturing) respond to an anisotropic environment in which the groundwater flows are linked to the fracture zones that are being limited by the fillings contained therein. In general, pit 1 is established as a means of low primary permeability and that presents numerous discontinuities (fractures) and alteration bands that determine the hydraulic properties, such as a limited secondary permeability. The Cretaceous calcareous aquifer is characterised by dominion of fractures that were later developed by karstification. This process did not develop in a similar way throughout the massif, since it is observed areas where it is more notable and therefore the rock acquires a greater permeability, specifically this can be noticed in the sector located between the plant and la garita (the sentry box).

The geological formations are recharged essentially by infiltration of rainwater. There are different factors that condition the recharge rate due to infiltration, such as: type of soil, vertical permeability of the underlying rock, slope of the land and existence of exoformas as dolines, sinkholes, etc. It should be noted that the results of the infiltration coefficient show very clear two opposite tendencies. One linked to materials with low infiltration capacity and secondly the development of epikarst areas. The observed epikarst are linked with outcrops of karstified limestone of Cretaceous age that act as a regulator of the recharge process on this formation. By plotting piezometric data versus precipitation, a delay can be observed between precipitation and effective infiltration in the deeper saturated zone of the Cretaceous aquifer. It is here that the regulatory effect of the epikarst can be observed.

In the areas where low to very low permeability formations appear, the infiltration coefficients take values of less than 15%, while in the areas of epikarst values of between 45-54% have been established. In general, the recharge zone of the hydrogeological system is located in Cerro Candela and in the SW of the concentrator plant (area where a highly karstified zone is located). The natural discharge of the hydrogeological system of the study area occurs towards the channels of the Hualgayoc and Tingo River and the springs associated with these channels and the Qda. Mesa de Plata. The most representative springs show a seasonal pattern in response to the most superficial zone of the saturated bands, related to the Cretaceous formation of karstified limestone, meaning that its flow increases rapidly after episodes of precipitation and rapid depletion, typical in a system of high diffusivity and low inertia.

A current condition of the hydrogeological system of the area is that they are influenced by the underground mining works existing in the area, prior to the pit 1. This network of galleries and tunnels cause the drainage of the geological formations, giving rise to discharges towards the channels in favour of the exit tunnels. Also, mine drainage work being carried out in the mining pit 1 are causing an alteration in the hydrogeological system, for whose characterization has been developed a numerical flow model. Another element that conditions the discharge of the hydrogeological system is associated with the drainage systems located in the south-eastern area of the tailings dam (UCB). In Figure 2, the conceptual model of the system studied is shown.


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Figure 2 | Conceptual model

The APLIS method was used to determine the infiltration coefficient. This method combines the typical characteristics of the study area according to five variables: altitude (A), slope (P), lithology (L), soil type (S) and infiltration (I). For each variable, different categories valued between 0 and 10 have been established, through a geographic information system (GIS). The coefficient is calculated by superimposing these variables in layers, allowing to obtain spatial distribution. Due to the lack of detailed soil data in the study area, the Soil Map of Peru was used (Cervantes, 2011). In this map, they characterize the study area as a Eutric Cambisol soil, to which a value of 5 points is assigned to the entire study area. In terms of absorption/infiltration forms (parameter I), a greater value has been given to fault zones, areas with higher karstification and geomorphological forms typical of karst, such as caves, dolines and sinkholes. Thus, a value between 6 and 8 has been given to the most karstified and fractured area, located between the La Planta and Cerro Candela sectors and a value of 1 to the least fractured and karstified areas. Finally, a distribution map is obtained of the infiltration coefficient to which the calculated excess rainfall is applied. In this sense, the amount of water that recharges the hydrogeological system on a monthly basis and the average annual recharge is calculated. In Figure 3, the infiltration coefficients obtained using the APLIS method are shown.


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Figure 3 | Infiltration coefficients

Mathematical flow model In order to know the hydrogeological functioning of the system, mathematical modeling was performed using

the FEFLOW software (Trefry & Muffels 2007; Diersch 2014), which uses the finite element method to solve the flow equation. The first simulation was to reproduce the flow model in permanent regime, prior to the mining operation. In other words, the operation of the hydrogeological system was simulated under natural conditions. Once the previous model had been calibrated, the model was generated under a transitory regime, based on the first model. The transitional model will allow us to carry out future simulations according to future needs. In Figure 4(a) Hydraulic conductivities, 4(b) System fractures, 4(c) FEFLOW model, 4(d) Pit drainage.

Figure 4 | Mathematical flow model. (a) Hydraulic conductivities, (b) Fractures of the system, (c) Model FEFLOW, (d) Pit drainage.

RESULTS AND DISCUSSION


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Regarding the resulting piezometry, two circumstances are observed: (a) The impact of the karstic structures in the upper and middle Yumagual zone, which function as a drainage, favoring the appearance of a depressed area in the piezometry close to La Planta. This zone is associated with a group of fractures (La Garita-Gran Cañón) that facilitate the saturated underground flow and the development of the superficial epikrast, conditioning the flow and drainage of the recharge in its superficial part through the karst structures towards the springs of the Hualgayoc River. (b) Underground drainage from the galleries and mines of the Carolina Mine. This model output has been approximated based on information provided by the mining operation. Very strong gradients are observed due to the deep excavation and the low permeability of the adjacent rock.

The calibration of the model in permanent regime was made considering the piezometric surface in the period April-May 2012. In Figure 5, it is observed how most of the piezometers are within the confidence interval, having obtained an error percentage in the 6% of modeling. The error has considered acceptable in this type of media so heterogeneous (fractured and karstified hydrogeological system).

Figure 5 | Permanent regimen calibration Figure 6 | Pumping test curves

Calibration of the model in transitory regime A monthly time step was simulated for a time period from January 1, 2010 to May 1, 2012. The piezometry

resulting from the numerical model in permanent regimen was used as the starting point for the calibration in transient regimen. For calibration, the piezometric evolution of the observation points was used. As a practice for the control of sensitivity in fringes and fractures, the effect of the 30-day pumping test performed on well PP-03 was recreated (Fig. 6).

The time series versus pump flow rate model was entered between January 2008 and May 2012 to visualize its transient effects on the control point levels. Due to the high degree of heterogeneity in the fractured medium of the pit, it is considered that the important thing in a calibration of this type is to represent in a general way the tendencies in the water levels. The simulated levels of the HPI-4-I piezometer are shown in Figure 7 (a). The fractured medium shows a quite significant oscillation of levels, which the numerical model dampens. These oscillations are linked to the combined effects of the pumping in the pit and the seasonality of the recharge. In the second half of the simulation there is no data to compare, however the result seems reasonable following the seasonality. Similarly, in the HPI-8-I piezometer (Fig. 7(b)), the downward trend is adequately represented, however, as in the previous result, the model generates a certain damping of the piezometric levels, with respect to the measurements actually recorded.


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Figure 7 | (a) Piezometer transient levels HPI-4-I Figure 7 | (b) Piezometer transient levels HPI-8-I

In the Yumagual formation, due to the importance of controlling the flows of Hualgayoc springs, the nivel of the middle and upper limestones in the Yumagual formation were compared. The piezometer PPT-9 middle part and HPI-7-I located near the top of the process plant has a very good calibration (Fig. 8 (a) and 8 (b)). It has one of the most complete series, covering almost four years of monitoring. Because the level is only influenced by the infiltration recharge, it exhibits the characteristic seasonal oscillations that are clearly seen in the measurements. The model presents a very good transient calibration.

Figure 8 | (a) Piezometer transient levels PPT-9 Figure 8 | (b) Piezometer transient levels HPI-7-I

Predictive flow simulation The mining unit provided information about its mining plan, in order to simulate the drainage of the pit for the

year 2013. The objective was to reduce the piezometric level to the level 3750 meters above sea level by the end of the year. A preliminary simulation was carried out, as the first approximation of the flow rates necessary to drain the pit, imposing a constant load-bearing edge condition on the model at the rate of mining. The condition was applied 10 meters below the pit level. The following scenarios were proposed: (a) Scenario 1: Pumping wells in the pit pump a flow rate similar to that calculated by simulation with constant potentials (6 to 12 l/s; Fig. 9). (b) Scenario 2: The pumping wells existing in the pit pump a flow rate equal to double that was calculated by simulation with constant potentials (12 to 24 l/s; Fig. 10). (c) Scenario 3: The pumping wells in the pit pump a flow rate equal to twice that calculated by simulation with constant potentials (12 to 24 l/s) and the recharge is considered zero (Fig. 11).

Figure 9 | Scenario 1: Evolution of simulated levels Figure 10 | Scenario 2: Evolution of simulated levels


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Figure 11 | Scenario 3: Evolution of simulated levels

In scenario 1, after simulation, the typical pumping cones were generated in the areas closest to the wells. However, laterally the descents produced are quite small, this is due to the fact that the rock mass (non-fractured diorite) has low permeability. In Scenario 2, pumping cones were also generated in the area adjacent to the drainage wells and a rather small lateral extension of the descents, due to the low permeability of the massif (non-fractured diorite). In Scenario 3, despite the fact that the pumping rates considered are high, it is not possible to depress the pit to the required levels, the numerical model offers the image of an intrusive body with slightly fractured blocks whose attenuated response to the effects of drainage makes it difficult to desaturate these areas. It is evident that the drainage wells should be located in fractured zones and it is necessary to analyze if between the less fractured blocks there are differences of permeability, since this is critical to evaluate the techniques for the desaturation of the rocks of the pit and their influence on the geomechanical response during the excavation of the slopes.

3 CONCLUSIONS

The conceptual model in the study area established as the main recharge zones in the Cerro Candela and

the SO of the concentrator plant, due to its high karstification. In relation to the discharge of groundwater, the

channels of the Hualgayoc and Tingo rivers were identified as natural drains that discharge the system, as well

as the springs associated with these channels and the Quebrada Mesa de Plata. On the other hand, a non-

natural discharge zone was identified, influenced by the existing underground mining works prior to the pit 1.

In the area, 4 hydrogeological units were identified, determined from their behavior and their hydrogeological

parameters. The following hydrogeological limits were assigned: (a) The Hualgayoc and Tingo rivers as

drainage axes, due to the fact that most of the springs are linked to the banks of these channels. (b) The

hydrological divide of the Mesa de Plata Creek to the east, because it behaves as a NO flow limit since it is

associated with very low permeability formations of the Fm. Chulec. (c) The fracture zone of Coymolache to the

west, considered as an open boundary.

The three-dimensional water flow model of the study area was constructed and calibrated for permanent

regime (natural conditions) and transitory regime (for a mining plan). Once the model had been calibrated, a

global drainage flow of 6 to 12 l/s was established using the numerical solution of constant loads, which will

ensure a piezometric level 10 m below the mining plan. Despite establishing an overall drainage flow, this

condition had to be simulated using point edge conditions, through the pumping wells. Three scenarios were

simulated for a period of one year; the first two scenarios correspond to a recharge condition, corresponding to

the wettest year recorded, as the most unfavourable scenarios. The third scenario was represented with a zero

recharge condition.

In the first scenario, a flow rate of 6 to 12 l/s was simulated (the flow rate established by constant load), in the second scenario, flow rates of 12 to 24 l/s were simulated (twice the flow rate established by constant load) and the third scenario simulated extraction flows of 12 to 24 l / s (this scenario considered zero recharge). For all three scenarios it is observed that the water table cannot be depressed below the levels of the pit. The numerical model reflects the reality of an intrusive body with very low permeability zones and therefore very difficult to drain. The model in the scenarios simulates the emptying in the fractured zones but not in the less fractured intrusive zones.

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