Water quality in relation to geo-ecosystem properties. A case study from
a jalca area near Cajamarca (Peru).
Cammeraat LH1, Seijmonsbergen AC1, Sevink J1, Hoogzaad YPG1, Stoops WS1, De
Vet SJ1, De Vries ME1, Van Veelen M1, Weiler HA1, Weiss N1, Sánchez-Vega I2,
Chunga-Castro F3.
1 Institute for Biodoversity and Ecosystem Dynamics, University of Amsterdam,
Science Park 904, 1098 XH, Amsterdam, the Netherlands
2 Escuela de Postgrado, Herbario CPUN, Universidad Nacional de Cajamarca,
Cajamarca, Peru
3 CIPDER, Los Naranjos 151 Urb El Ingenio, Cajamarca, Peru
Abstract
This study gives a first approximation of the relationship between hydrology and
water quality, geology and soils for a jalca ecosystem near Cajamarca in Northern
Peru. As the jalca areas are prone to degradation of their humic soils as a result of
land use and climate change, more information is needed on their hydrology and geoecology. During two field campaigns the area was surveyed with regard to geology,
geomorphology, soils, and vegetation. Furthermore, hydrological routings were
executed, measuring electrical conductivity (EC25) of stream waters and selectively
sampling these waters.
Soils showed a clear relationship with their substrate and were in many cases high in
organic material as shown by the dominance of mollic and umbric A horizons,
respectively. The hydrology is driven by a water surplus of around 300mm per year,
and stream discharge is closely related to the seasonality of the rainfall.
We found that there is a clear relationship between the underlying substrate and the
water chemistry of the streams and fens. Two set of water types can be distinguished:
one originating from dacitic extrusive rocks (ignimbrites), with extremely low total
dissolved solid levels, a dominance of Ca2+ and HCO3-, and relatively high sodium
levels, as expressed by high SAR (sodium adsorption ratio) values. The high SAR
values make this water risky for application in irrigation. The other type of water
originates from limestone dominated bedrock areas, showing much higher total
dissolved solid levels, and low SAR levels. Chemical water pollution, as expressed by
nitrates and ammonium, was especially related to settlements.
Key words
Ignimbrites, Soils, Hydrology, Electrical conductivity, Sodium adsorption ratio
Introduction
Other than the Andean páramo ecosystems of Colombia and most of Ecuador, their
more southern Andean equivalents (páramo’s and jalca’s of Peru and Southern
Ecuador) are marked by a much more varied geology and a far wider range of soils
when compared to the predominantly volcanic rocks and deep volcanic ash soils of
the northern Andean ecosystems. Many of these non-volcanic soils lack the high
water retention capacity and permeability of the characteristic northern Andean
Andosols. Moreover, climate is often drier and/or more seasonal, such as in the
southern jalca-type ecosystems. As a consequence, regional and even local variation
in discharge characteristics (quantity and composition) may be considerable.
Understanding the hydrological characteristics and behaviour at catchment level is
crucial for appropriate management of these high Andean catchments. Unfortunately,
contrary to than their northern equivalents, baseline studies on their hydrology,
geomorphology and soils are truly scarce. This note concerns a case study on the
PPA-site near Cajamarca, an area with a moderate population density which has
highly varied geological and soil patterns and jalca-type ecosystems. It is focusing on
the hydrology of this area and first sets out to provide a background on relevant
landscape aspects (including soils, geomorphology and climate) followed by a more
detailed analysis of the hydrological characteristics of the area.
General information
The area of interest is located in the province of Cajamarca in Peru (7°10’ S, 78°36’
W; figure 1) on the continental divide at an altitude between 3400 and 4000 metres.
The central part of the area is formed by an undulating broad high plain, which at
places is surrounded by and connected to higher mountainous areas, notably in the
NW. The plateau is dissected by deep river valleys draining either into the graben of
Cajamarca (Rio Cajamarca) and eventually to the Atlantic, or steeply into western
direction to the nearby Pacific (Rio Chilete, Jequetepeque basin).
Figure 1. Location map of the fieldwork area.
The results presented in this study were obtained during an international MSc course
on applied geoecology, carried out in June-July 2008 and 2010 by two groups of
students originating from Peru, the USA and the Netherlands. The course was jointly
organised by the University of Amsterdam, the Universidad Nacional Cajamara and
CIPDE.
Geology and geomorphological processes
Three different substrates dominate in the area of interest (Mapa geológico del
Cuadrángulo de Cajamarca 19):
1. The basement of the area is formed by folded Mesozoic shallow marine
formations. They are mainly composed of alternating limestone, marl, quartzite,
and sandstone (Ramos, 1999) of which the Cretaceous limestones of the
Cajamarca formation and quartzites of the Farrat formation are particularly
resistant to weathering and erosion.
2. Granitic rocks, mostly Na-feldspar rich tonalites, have intruded into these
basement rocks in the late Lower Tertiary (Reyes-Rivera 1980).
3. Extrusive rocks (San Pablo formation, upper Calipuy Group), mainly consisting
of well-banked and poorly sorted pyroclastic rocks, so-called ignimbrites, were
discordantly deposited on top of the folded Mesozoic rocks in the late Lower
Tertiary. They have a dacitic composition (Reyes-Rivera 1980). These extrusive
rocks are probably related to the intrusion of the igneous rocks mentioned under
2), as suggested by Reyes-Rivera. Such a relation between granitic intrusions and
ignimbrite deposits was clearly demonstrated in an area 60 kilometres south of
Cajamarca (Navarro & Rivera 2006).
Figure 2. Simplified geological cross section showing the main geological units and
their stratigraphic position.
The morphology of the study area is dominated by the resistant folded Cretaceous
geological formations that form the backbone of the landscape, providing a threshold
for incision at the rim of the high plain. The central, flatter parts are covered by an
undulating sheet of ignimbrites, which discordantly covers the underlying folded
rocks. In many cases these underlying rocks have become exposed and jut out through
the ignimbritic cover. At several places, the ignimbrites exhibit a spectacular tower
karst (e.g. Cumbemajo).
Glaciers repeatedly covered most of the area over 3000 m asl during the various ice
ages, the latest ending around 12000 years ago (Birkeland et al 1989). Presumed
remains of such glacial cover in the form of basal till and recessional moraines were
observed at 3500m asl in the area around Sexemayo and are probably related to a
relatively recent major glacial extension. Current geomorphological processes include
natural slope processes amongst which particularly rock fall, while rill, tillage and
gully erosion are often induced by inappropriate agricultural land use. Furthermore
karst processes are important in limestone areas, but also silica karst is prominent in
the ignimbrites. Fluvial processes currently induce the incision of streams and
drainage networks, which are expanding into the upper parts of the plateau-like upper
catchments, by backward erosion processes.
Soils
The range and variety of soil types is much larger than in the more northerly páramo
regions of Colombia and Ecuador. This is due to the absence of the thick blanket of
recent ash layers that induces a very homogeneous soil cover, so characteristic for
large parts of Ecuador and Colombia.
The soils found in the Cajamarca area are strongly related to their parent material. A
characteristic set of catenas is given in figure 3, showing the dependence of soils on
both topographical position and parent material.
Figure 3. Spatial distribution of soils along characteristic catenas on ignimbrites and
quartzites (after Hoogzaad et al 2008). Soils were classified according the IUSS-WRB
(2006).
On the quartizitic rocks and their derived slope deposits poorly developed soils like
Leptosols, Cambisols, and Regosols prevail, all with light coloured Ah horizons
(ochric horizons). On carbonate rocks like limestones, Leptosols are prominent in the
top positions, and Phaeozems and Vertisols are more common on gentle sloping parts
of the catena, both having a very dark mollic Ah horizon. The ignimbritic and granitic
rocks show completely different soils. On top and hill slope positions, soils have very
dark umbric Ah horizons and are classified as Umbrisols or, when more than 5%
volcanic glass is present, as vitric Andosols. The more recent deposits are
characterized by poorly developed soils like Regosols, or in the valley bottoms
Gleysols or Histosols.
Table 1. Overview of main soil types in relation to bedrock and topographical
position.
Parent material
Topographical
position
Top
Quartzites
Leptosol
Limestones/
Marls
Leptosols
Hill slope
Leptosol/
Cambisol
Phaeozems/
Vertisols
Valley bottom
Regosol
Phaeozems /
Vertisols /
Gleysol
Ignimbrites
Granitic rocks
Leptosol /
Andosol /
Umbrisol
Andosol /
Umbrisol
Leptosol /
Umbrisol
Gleysol /
Histosol
Leptosol /
Umbrisol/
Planosol
Gleysol/
Histosol
Quaternary
sediment
Regosol
-
Gleysol /
Histosol
The surveys also showed that the ignimbritic soils were often shallower and more
sensitive to degradation as a result of land use, whereas the soils on limestone in
general were deeper and had better soil structure. Lastly, it should be mentioned that
on tonalite incidentally Solodic Planosols were observed. These soils are
characteristic for substrates that have significant amounts of readily weatherable Nasilicates and were formed under xeric climatic conditions (seasonal precipitation),
similar to the Vertisols in limestone/marl. Solodic Planosols may have relatively high
pH (up to 9 and higher) and high Na-saturation in the subsoil and are marked by their
highly stagnative B-horizon.
Climate
Because of the high altitude, the climate of the region is characterized by relative low
annual temperatures, which vary little over the year (13°C for Cajamarca) with a
diurnal amplitude of about 17°C (values for Cajamarca at 2650m altitude, De la Cruz
et al 1999). Temperature decreases with altitude. In figure 4 the temperature gradient
is shown for the nearby transect Cajamarca-Yanacocha. No data exist for the field
area.
Figure 4. Rainfall and temperature in relation to altitude for the stations Augusto
Weberbauer (near Cajamarca), Granja Porchon, La Quinua, Yanacocha, Maqui Maqui
and Carachugo (from low to high altitude). Data source: Stratus Consulting (2006)
Rainfall has not been measured in the area itself, but the data presented in figure 4
show a clear increase of rainfall with altitude, although the impact of exposition and
position with respect to rain shadow is rather important and explains the lower
correlation. The rainfall data suggest that rainfall in the area of study should be
around 1100 mm per year. Rainfall is unevenly distributed over the year, with a
relatively dry winter period from June to August and common rains in the remainder
of the year with the wettest period from December to March (Stratus Consulting
2006). This pattern is also visible in figure 5, which presents data for the Huacraruco
catchment south of Cajamarca.
Figure 5. Precipitation over the years 2004-2007 at the Huacraruco site measured with
a rudimentary rain gauge. Missing bars represent hiatuses in observations (from van
Veelen & de Vet 2008).
Hydrology
Potential evaporation does not show a large variation for the four higher stations used
in figure 4 and is about 2.3mm /day (Stratus Consulting 2006). Based on these
figures, an annual surplus of around 300mm /yr would occur. Such relatively large
annual surplus explains the common occurrence of springs, permanent streams, and
peat bogs in the area of study.
Springs are particularly found at the contact between the porous ignimbritic rocks and
underlying impervious formations (marls) or clayey weathering residues (e.g. in
limestone). They occur at the base of cliffs where ignimbrites crop out and often
exhibit prominent silicate karst. Furthermore, soils in ignimbrite often have iron
hardpans at the base of the solum, i.e. at their transition to unweathered bedrock. Such
soils, commonly found on ignimbrite plateaus, give rise to seasonal water stagnation
and to small streams that are mostly intermittent (wet season) but in places even may
be permanent.
The limestone areas behave hydrologically differently, as they have fewer permanent
streams because of the high permeability of the limestones and their soils. When
streams are present, they often originate at a lithological contact with impervious
layers such as marls or shales. Springs in the limestone area seemingly have a higher
and more constant base flow, which can be attributed to their feeding by karst
groundwater systems.
Granitic rocks and quartzites have in common that groundwater reservoirs in these
rocks are limited to cracks and fissures, which are mostly concentrated in the upper
metres and are of limited size. Base flow therefore is low and streams react relatively
rapidly to precipitation, the storage capacity of these formations being low in
comparison to the ignimbrites and limestones.
The valley bottoms are commonly very wet, as impervious materials underlie them.
These include clayey weathered bedrock and impervious parent materials such as
shales and marls, hardpans or sub-glacial till. In most valley bottoms, a permanent
shallow water table is present and many waterlogged areas, peat bogs and small lakes
exist. Aquifers are often exploited for local irrigation, especially in the Jamcate
catchment. Soils in these wet environments are commonly Histosols or Gleysols,
having thick Ah and B-horizons that contain considerable amounts of organic matter
(up to 400 ton ha-1; Cammeraat et al 2011). These carbon stocks can only survive if
the water tables are being maintained at their current level, as these soils are prone to
oxidation upon drainage. Buytaert et al (2006) discusses the threats for these organic
rich soils in the páramos, such as drainage, land use change, climate change and
tapping of water for irrigation.
For the area under study quantitative hydrological information is not available.
Quantification of both the input and output terms of the hydrological system is highly
needed, if one would intend to develop and implement management schemes to
preserve the water and soil carbon stocks. As a first step towards such quantification,
a reconnaissance survey was carried out by studying water quality in permanent
streams and lakes, in relation to geology and land use during the two winter field
campaigns in 2008 and 2010. In figure 6 the hydrological routings carried out during
both campaigns are given.
Hydrological routings are ideal to get a quick impression of the total amount of
dissolved solids in river water by measuring its electrical conductivity. A simple
sensor can directly detect changes in conductivity. Normally there is a direct
relationship between the electrical conductivity and the amount of dissolved ions in
the stream water (1 mmol/l ∑ cations or ∑anions ≈ 100 μS/cm). It enables also the
possibility to selectively sample water on points of interest as localized during the
hydrological routing. Electrical conductivity readings were carried out every 100
metres and were automatically corrected for temperature to a reference temperature of
25oC.
Figure 6. Results of the hydrological water quality routing of 2010 (modified from de
Vries and Weiss 2010).
Measured electrical conductivity (EC25), as obtained during the routing in 2010, is
projected on top of the geological map in Fig. 6. A clear relationship between the
geology and the EC25 is visible. The limestone areas show clearly higher EC25 values
when compared to the values found for streams originating in the granitic, ignimbritic
and quartzite lithologies. Thus, river A in figure 6 originates in ignimbrite bedrock
and has water with very low amounts of dissolved solids, as expressed by an EC25 of
less than 100 µS/cm over the whole stream section. Ignimbrites consist of silicate
minerals that do not so easily dissolve and the flow path of the water through the
ignimbrite body is rather short. By contrast, stream B shows much higher amounts of
dissolved solids with values between 400 and 500 µS/cm in the upstream section.
Values decrease downstream where the river flows through marls or shales, probably
related to mixing of limestone-derived water with water with lower dissolved solids
contents derived from the marls. The values are 3 to 4 times higher than in ignimbrite
streams such as river A, which is in conformance with the literature (Appelo and
Postma 2006). River C shows a very nice example of mixing of water from two
different sources. The source of the Rio Cumbemayo is in the limestone and marl area
at the watershed and it subsequently flows through a large area with ignimbrites,
which discharges water with very low EC25. At the lowest measured section, the
contribution of the limestone area is strongly diluted and not visible anymore in the
EC25.
Figure 7, based on the measurements taken in 2008 shows the same relationships
plotted against down stream distance of the source, although the absolute values
differ. Values measured in 2010 were consequently lower, and although some land
use change effects may be involved, the major difference between 2008 and 2010 is
probably related to different hydrological conditions, as it is well known that the EC25
strongly depends on stream discharge and residence time of water in the subsurface.
Higher discharges result from higher amounts of direct precipitation and show a
relative large contribution of water that is moving close to the surface, with a relative
short residence and contact time, and hence contain a lower amount of dissolved
solids. Low discharges are mostly fed by deep and slow moving water, with longer
residence time and hence higher amounts of dissolved matter (Appelo and Postma,
2006). Although we do not have data on stream discharge, we know that the dry
period in 2010 started relatively late (middle of June). Land use effects, especially
land clearing, would have increased the total dissolved solids while the opposite is
observed, however no clear change was observed in land clearings or land use change
between 2008 and 2010.
Figure 7. EC routings of 2008, also showing clear relationships between EC and
bedrock. All limestone-derived streams have higher EC’s when compared to the
ignimbrite streams (from: Van Veelen and De Vet, 2008).
With respect to water pollution it can be stated that the ignimbrite waters are low in
dissolved material and thus agricultural or urban pollution will show up very clearly
and quickly. Even minor pollution will have a significant impact since these waters
have hardly any chemical buffering capacity. In water originating from limestone,
pollution will be more obscured, these waters being better buffered. The differences
in chemical composition and buffering capacity are evident from table 2.
Table 2. Water chemistry at 4 characteristic sampling points. Codes A-D correspond
to the streams indicated in fig. 5, except for B. All concentrations in μmol/l, except
for pH, SAR (-) and EC25 (μS/cm). DOC = dissolved organic carbon, SAR= sodium
adsorption ratio, n.d.= not determined. (data source: van Veelen & de Vet 2008).
pH
EC25
DOC
SAR
HCO3SO42ClNO3PO43-
A
7.55
173
793
6.8
1400
24
69
11
4.5
B*
7.88
590
916
1.4
3278
310
49
41
0.1
C
7.25
94
347
12.6
780
10
27
10
6
D
7.71
180
572
9.0
1589
108
34
5
0.7
K+
Na+
Ca2+
Mg2+
NH4+
Al3+
Fe2+
Sr2+
H4SiO4
A
B*
183
25
167
70
536 2128
70 454
44 <10
1.82 0.52
0.61 0.07
2.41 5.32
466
87
C
109
192
177
56
<10
3.05
1.5
1.36
853
D
34
222
526
79
66
n.d.
0.34
1.7
261
* this sample was not taken in stream B in figure 5, but in a comparable limestone stream
Table 2 displays the general chemical characteristics for four typical types of water:
B is typical for water composition of streams originating in limestone areas, with high
Ca2+ and HCO3- concentrations and low in dissolved silica. C and also A are typical
for water with their source in ignimbrite environments, with very low levels of
dissolved solids, with Ca2+ still as the dominant cation and HCO3- as the dominant
anion, and very high levels of dissolved silica. The relative high concentration of Na+
and K+ is striking as well, which must be derived from the dacitic ignimbrites,
containing considerable amounts of Na- and Ca-feldspars. These elements occur
much less in the streams that originate from limestone. Cooper et al (2010) found also
a dominance of Ca2+ and HCO3- dominate surface waters and in some cases also high
Na+ and K+ levels for streams NE of Cajamarca. However these authors did link
hydrochemistry to lithology. Stream D shows mixed water sources.
The SAR values of the stream water are rather high for streams A, C and D. The
Sodium adsorption ratio (SAR) is the ratio of the amount of sodium ions (Na) over the
square root of the amount of Calcium and Magnesium ions in the solution. If clays in
the soil contain 15% or more Na+ at their exchangeable sites then this can destabilize
soil aggregation upon wetting. If water is applied to the soil with a relatively high
sodium content, then Na+ will replace the Ca2+ and Mg2+ at the clay exchange
complex and make the soil aggregates unstable (Seelig, 2000). This may lead to soil
crusting and reduced infiltration and dispersion of clay, eventually leading to erosion
problems. The high SAR values in combination with the low EC25 of the studied
water of stream A, C and D could cause problems if used for irrigation. They are
classified as having a moderate to severe SAR/Salinity hazard (Lenntech, 2011) if
applied to soils, and therefore application may induce dispersion and erosion of
topsoils.
DOC levels are quite high for all waters, which is related to the high organic carbon
levels in the valley bottom soils. Nitrate and ammonium levels were quite low,
although human impact on stream water quality was still discernible. In some of the
samples, however, the level of pollution was considerable, but they diluted
downstream. Measurements of the biological quality of the streams (e.g. microbial
activity) were outside the scope of the 2008 and 2010 field studies.
Systematic comparison of the values found to those observed in the catchments of
northern Ecuador and Colombia with their characteristic Andosols is problematic,
since very few papers have been published on stream chemistry of the latter páramo
catchments. Nevertheless, the differences in pH and amounts of dissolved solids,
particularly for monovalent cations (K and Na) seem to be considerable, pH values
reported for northern páramo waters generally being distinctly lower than 7. They can
be attributed to the specific mineralogy (Na-rich silicates) in combination with the
clear climatic seasonality and are also reflected in the occurrence of such soils as
Vertisols on the limestones and some rare Solodic Planosols on tonalite.
Effects of pollution on water quality were analyzed by van Veelen & de Vet using a
spatial regression between water quality and settlement pressure (proximity of
settlements to streams).
Figure 8. Settlement pressure for the three catchments in this study is indicated in
background shades (modified from van Veelen and de Vet 2008). Electrical
conductivity (EC25) of the streams is indicated by the colour of the lines depicting the
flow paths of the streams: red indicates higher EC25’s and green lower EC25’s.
They used multiple-ringed buffers around settlements to determine the distance to the
settlements, as shown by the background colours of figure 8. The water quality of the
streams is projected on top of the settlement pressure zones. Segments of streams are
depicted as coloured lines, where warmer colours (red-orange) indicate higher EC25
values and greener tones indicate water with lower EC25 values. From figure 8 it
becomes clear that the streams near the settlements have higher electrical
conductivities, when compared to locations further away from settlements. Statistical
analysis proved this relationship to be significant. This can be interpreted as a
stronger effect on water quality closer to settlements than in stream segments further
away from settlements, where human influence is much less and polluted waters are
diluted. Hence, water quality is influenced stronger by local socio-economic
activities, especially those close to, or in the settlements.
Conclusions
Little is known about the geoecology and hydrology of jalca geo-ecosystems. In this
note some insights are given on the lithology, geomorphology, and soils of this area,
showing that there is a close relationship between lithology and geomorphic position,
and main soil types. The influence of glacial, fluvial and karst processes is notable in
the area, also on ignimbritic rocks.
1. The lithological differences are not only reflected in the soils but also in the
hydrology and drainage water chemistry:
- The limestone areas have fewer springs and more subsurface drainage than the
areas with ignimbritic bedrock. Because of their different lithology also the
discharge response to rainfall will be different.
- Limestone areas show much higher dissolved solid loads, dominated by Ca2+ and
bicarbonate. The water from the ignimbritic rocks shows much lower total
dissolved solid loads. Although Ca2+ and HCO3- are also dominant, it also
contains relatively high amounts of K+ and Na+ as well as dissolved silica.
- Pronounced seasonality and parent materials containing fair amounts of Nasilicates together induce relatively high pH and Na-values, strongly deviating
from values encountered in catchments of northern Ecuador and Colombia with
humid páramos and dominated by soils in recent volcanic ash.
- The combination of high SAR values and low EC of the stream water derived
from the ignimbrite areas, may induce salinity problems if applied on soils, and
could accelerate soil degradation and erosion.
2. In places, the water is polluted with nitrates and ammonium of anthropogenic
origin. Pollution patterns could be related to settlement pressure, evidencing the
importance of appropriate land use and its planning for the conservation of the
essential water resources.
Acknowledgements
Thanks are due to the Postgraduate School of the Universidad Nacional de Cajamarca
and especially Dr. Nilton Deza Arroyo and Dr. Pedro Ortiz-Oblitaz for helping
organizing this course and providing access to the their laboratories as well as
offering transport, CIPDER and the PPA team Cajamarca/PPA team Peru, (Carlos
Cerdán, Gaby Lopez, Santos Cotrina, Alex Chavez, Jorge Rechartes) for their help
during the field campaign as well as during the preparation, the communities of
Sexemayo, Jamcate and Huacraruco for their hospitality and for being so kind for
granting access to their territories, and IBED for providing financial support and for
carrying out laboratory analyses and finally, Manuel Roncal Rabanal "Lito" and all
the students taking part in the course of applied geo-ecology in 2008 and 2010.
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