İzmit havzasının hidrojeoloji incelemesi ve yeraltısuyu akım modellemesi
An examination of the hydrogeology of the İzmit basin, and modelling of the groundwater flow
Yazar: MERAL ERDOĞAN TOPÇUOĞLU
Danışman: PROF. DR. REMZİ KARAGÜZEL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Jeoloji Mühendisliği Ana Bilim Dalı / Jeoloji Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: Jeoloji Mühendisliği = Geological Engineering
Dizin: Hidrojeokimya = Hydrogeochemistry ; Hidrojeoloji = Hydrogeology ; Su jeokimyası = Water geochemistry ; Yer altı su akımı = Ground water flow

Özet:
Bu çalışma; İTÜ Bilimsel Araştırma Projeleri Birimince desteklenmiştir. Doktora tezinin amacı böylesine zor hidrojeolojik koşulların egemen olduğu İzmit Havzasında, yeraltısuyunun yönetilmesine yönelik hidrojeolojik incelemelerin yapılması ve YAS akım modelinin oluşturulmasıdır. Bir başka ifade ile İzmit Havzası hidrojeoloji incelemesi kapsamında taneli akiferde kontrolsüz yeraltısuyu çekimine bağlı seviye düşümleri sonucunda Sapanca Gölü ile olası etkileşim için  kritik çekim miktarının (eşiğin) belirlenmesi hedeflenmiştir.  Arazi çalışmalarına başlamadan önce İzmit Havzası ve yakın çevresinde daha önceden yapılmış çalışmalar literatür araştırması kapsamında derlenmiş ve bölge hakkında bilgiler edinilmiştir. Ekim 2016 ve Mayıs 2017 tarihlerinde arazi çalışmaları kapsamında havzanın jeolojik özellikleri incelenmiş, jeoloji haritası revize edilmiş, havzaya ait sayısal yükseklik modeli (DEM) oluşturulmuştur. İzmit Havzası, jeolojik ve tektonik özelliklerine göre Örtü Birimleri, İstanbul Zonu (Kuzey İstif) ve Armutlu-Almacık Zonu (Güney İstif) olmak üzere 3 başlık altında incelenmiştir. Paleozoyik istifler, arkoz-kumtaşı-silttaşı ardalanmalı akarsu birimleri olan Kurtköy Formasyonu ile başlar. Üzerinde uyumlu olarak kıyı-sığ denizel kuvars kumtaşı, çakıltaşı olan Aydos Formasyonu yer alır. İstif üste doğru lagün-şelf çökeli şeyl-kumtaşı ardalanmalı Gözdağ Formasyonuna geçer. Permo-Triyas yaşlı birim kumtaşı-çakıltaşı-çamurtaşı ardalanmalı Çakraz Formasyonudur. Triyas istifi üzerinde kırıntılarla başlayan Geç Kretase yaşlı çökelim izlenir. Bu bölgede Jura çökelimi yoktur. Kretase istifleri, kumtaşı-şeyl ardalanmalı Teksen Formasyonu ve Hereke Formasyonudur. Geç Kretase daha üst düzeylerde kireçtaşı ve killi kireçtaşı litolojilerden oluşan kalın Marn-kireçtaşı ardalanmalı Akveren Formasyonu ile temsil edilir. Eosen istifi ise kumtaşı-killi kireçtaşı ardalanmalı Çaycuma Formasyonu ve aglomera, tüf, andezit, bazalt ve volkanik kumtaşından oluşan Yığılca Formasyonundan oluşur. Yarımadada kıyı şeridi ve kıyı düzlüğü boyunca az tutturulmuş Pliyosen yaşlı kırıntılar (Örencik Formasyonu) ve Kuvaterner'e ait alüvyal çökeller ile yamaç molozu vardır. Armutlu Yarımadası'nda yer alan kaya stratigrafi birimleri iki ana grupta toplanabilir. Bunlardan birincisi düşük ya da nispi olarak daha yüksek dereceli metamorfik kayalar, diğeri ise metamorfik kayalar üzerinde Senomaniyen-Türoniyen yaşlı kireçtaşlarıyla uyumsuz olarak yer alan Kretase-Pliyosen yaş aralığındaki metamorfik olmayan magmatik ve çökel kayalardan oluşur. Armutlu-Almacık zonunda birbiriyle tektonik ilişkili olan Permiyen-Triyas yaşlı Sultaniye Metamorfitleri, Üst Jura-Alt Kretase yaşlı Keltepe Mermeri, Üst Kretase yaşlı Akçay Metamorfitleri bulunmaktadır. Bunlar üzerinde açısal uyumsuz olarak Üst Kampaniyen-Alt Eosen yaşlı Abant Formasyonu tarafından açısal uyumsuzlukla örtülür. Erken-Orta Eosen yaşlı Çaycuma Formasyonu ve Yığılca Formasyonları, Abant Formasyonu  ve diğer birimler üzerinde ise uyumsuzdur. 
Çalışma alanının içerisinde ve yakın çevresinde bulunan Devlet Meteoroloji İstasyonlarının (DMİ) uzun yıllara ait ölçümleri değerlendirilerek, havzanın bilanço elemanları ortalama yağış, potansiyel ve gerçek buharlaşma ile toplam akışa (yüzeysel ve yeraltı) geçen su miktarları hesaplanmıştır. İzmit Havzası'nda, ortalama yağış ve buharlaşma hesaplamalarında 6 adet devlet meteoroloji istasyonuna ek olarak, havzayı temsil ettiği düşünülen 4 adet İSAŞ meteoroloji istasyonu da değerlendirilmeye katılmıştır. Buna göre, eşbuharlaşma eğrileri yöntemi ile ARCGIS programı kullanılarak İzmit Havzası için ortalama buharlaşma değeri hesaplanmıştır. Havzadaki yüzeysel ve yeraltı akışının ayırımının gerçeğe yakın hesaplanması için SCS-CN yöntemi tercih edilmiştir. SCS-CN yöntemi kullanılarak Kocaeli Meteoroloji İstasyonunda ve Sakarya Meteoroloji İstasyonunda yüzeysel akış hesaplanmıştır. Alüvyon içerisinde resmi kurumlar ve özel kişilere ait çok sayıda sondaj kuyusu ve keson kuyu mevcuttur. Kuyular ile çekim, kaynaklar ile boşalım hesaplanmıştır. Sonuç olarak; ölçümlerde ve hesaplamalarda olabilecek hatalar dikkate alındığında emniyetli kullanılabilecek yeraltısuyu miktarı beslenme-boşalma farkının % 60'ı olarak hesaplanmıştır.
Arazi çalışmaları sırasında kayaç-su etkileşimi, birimlerin hidrojeolojik özelliklerinin ortaya konulması için havzadaki formasyonlar hidrojeolojik olarak da incelenmiştir. Su numunelerinin fiziksel karakterlerini uzun süre koruyamadıklarından dolayı sıcaklık (T), pH ve elektriksel iletkenlik (EC) gibi fiziksel özellikleri in-situ (yerinde) ölçülmüştür. Su numuneleri laboratuvardaki kimyasal analizler için uygun saklama ve taşıma koşullarında alınmıştır. Kocaeli Havzasında, 2016-Ekim ve 2017-Mayıs aylarında detaylı arazi çalışmaları sonucunda konumları belirlenen, havzayı temsil ettiği düşünülen lokasyonlardan 41 adet kurak döneme ve 35 adet yağışlı döneme ait su örneği alınmıştır. Havzadan temsili lokasyonlardan alınan 76 adet su numunesinin kimyasal analizi ACME Laboratuvarlarında; NO3 ve SO4 analizleri ise İstanbul Teknik Üniversitesi Hidrojeoloji laboratuvarında yapılmıştır. Suların kimyasal özelliklerine göre kalitelerini belirlemek için Schoeller Diyagramı, Piper Diyagramı, Durov Diyagramı, Gibbs Diyagramı ve Schoeller Yarı Logaritmik Diyagramı hazırlanmıştır. İzmit Havzasında en önemli kirlilik kaynağı yoğun olarak yapılan sanayi, tarımsal faaliyetler ve hayvancılıktır. Ayrıca, ovada yeraltısularının kirlilik tespiti için iz element analizleri (As, Cr, Cu, Fe, Mn, Ni, Zn) Acme (Kanada) Laboratuarında yaptırılmıştır. Ağır metal analiz sonuçları Türk İçme Suyu (TSE 266) ve Dünya Sağlık Örgütü (WHO) standartlarında müsaade edilen maksimum sınır değerlerin aşılmadığı tespit edilmiştir. 
Yeraltısuyunun kirlenmeye karşı hassasiyet haritalarının oluşturulması amacıyla USEPA (United States Environmental Protection Agency) için geliştirilen DRASTIC Model (DM) İzmit Havzasına uygulanmıştır. DRASTIC Model;  Yeraltısuyu Derinliği (D), Net Beslenme (R), Akifer Tipi (A), Toprak Sınıfı (S), Topoğrafya (T), Vadoz Zon Etkisi (I) ve Hidrolik İletkenlik (C) gibi jeolojik, hidrolojik ve hidrojeolojik parametreler ile yeraltısuyunun kirlenmeye karşı hassasiyet derecesinin belirlenmesini sağlayan bir yöntemdir. D, R, A, S, T, I ve C değişkenleri yeraltısuyu kirlenebilirliğine etki eden parametreleri temsil etmektedir. Sınıflandırma ve ağırlıklandırma işlemi yapılan tüm parametreler kullanılarak DRASTIC İndeks (Dİ) değeri hesaplanmıştır. Mayıs-2017 döneminde havzanın temsili noktalarından alınan su örneklerinde, özellikle alüvyon ortamda antropojen kökenli kirlenmeye işaret eden 10 mg/l üzerinde nitrat konsantrasyonu ölçülmüştür. Bu bulgular, yeraltısuyu kirlenme potansiyeli yüksek olan ve henüz üzerinde kirletici bulunmayan karbonatlı ortamların ve kirlenme belirtileri bulunan alüvyon ortamın özenle korunması gerektiğini göstermektedir. Yeraltısuyu kirliliğine karşı en hassas alanlar alüvyon, eski alüvyon yelpazesi, alüvyon yelpazesi, Örencik Formasyonu, güneyde bulunan mermer ve kireçtaşları belirgin olarak ortaya çıkmıştır. Havzanın güneyinden boşalan kaynaklar göle ulaşan dereleri beslemektedir. Mevcut koruma kuşaklarına ek olarak Sapanca Gölüne boşalan derelerin beslenme alanları ile bu derelerin üzerinde bulunduğu ve baz akımla beslendiği taneli ortamları da kapsayan "Hidrojeolojik Tabanlı Koruma Alanları" önerilmiştir. Haritada kirlenmeye karşı çok hassas ve hassas alanların sırasıyla alüvyal ortamlar ile karstik kireçtaşlarının kapladığı alanlara karşılık geldiği görülmüştür. Bu nedenle kaynak boşalımları ile doğrudan Sapanca Gölüne ulaşan dereyi oluşturan ve doğal arıtma süreçlerinin gelişmediği kireçtaşları ve mermerler ve alüvyon ortam "Hidrojeolojik Tabanlı Mutlak Koruma Alanı" olarak sınıflandırılmıştır Eski alüvyon yelpazesi, alüvyon yelpazesi ile Örencik Formasyonu ise doğrudan kaynak boşalımları gözlenmediği ve buna karşın doğal arıtma yeteneği olmasından dolayı "Hidrojeolojik Tabanlı I. Derece Koruma Alanı" olarak tanımlanmıştır. 
İzmit Havzasının yeraltısuyu akımının belirlenmesi için MODFLOW 2000 programını baz alan IMOD kullanılmıştır. Bu programı için jeolojik-hidrojeolojik veriler, yağış ve buharlaşma miktarı, akiferin beslenme miktarı, rasat kuyularındaki yeraltısuyu seviyeleri, işletme ve şahıs kuyularından çekilen debi miktarları kullanılmıştır. Sınır koşulları, model alanının jeolojik ve hidrojeolojik parametrelerine göre belirlendikten sonra, IMOD ile grid ağları oluşturularak tabaka tipleri, sayıları ve model için gerekli parametreler girilmiştir. IMOD kullanılarak İzmit havzasındaki 195 km2'lik bir alana sahip akiferde yeraltısuyu akım modeli çalışmaları yürütülmüştür. Akiferin, model alanı 100 m x100 m boyutunda 6 tabaka, 700 sütun ve 340 satırdan oluşan 238000 adet hücreye bölünerek modelin grid ağı oluşturulmuştur. Model çalışmasında, DSİ tarafından yapılan sondaj loglarına ait litolojik bilgiler kullanılarak hazırlanan jeolojik en kesitlere bağlı kalınmıştır. Yeraltısuyu akım modelinde akifer sisteminin sınır koşulları için, model alanının doğusunda yer alan Sapanca Gölü (30 m) ve batısında bulunan İzmit Körfezi (0 m) sabit seviyeli hidrolik yük (constant head boundary condition, CH) olarak tanımlanmıştır. Eşyağış eğrileri yöntemi ile ortalama yağış (1976-2017) model alanı için hesaplanmıştır. Net beslenim, yağıştan buharlaşma ve yüzeysel akış çıkarılarak hesaplanılmıştır. Modelde ayrıca kilin yüzeyde göründüğü kesimlerde beslenme 0,20 mm/gün olarak alınmıştır. Güneyde yer alan kaynakların toplam boşalım miktarı hesaplanmıştır. Model alanındaki rasat kuyularından ölçülmüş olan yeraltısuyu seviye değerleri kullanılarak, modelin kalibrasyonu yapılmıştır. Akım yönleri incelendiğinde İzmit Körfezinin batısında Marmara Denizine, Doğusunda ise Sapanca Gölüne doğru belirgin bir yeraltı suyu akışı olduğu gözlenmektedir. Model sonuçlarına göre İzmit Körfezi ile Sapanca Gölü arasında bir yeraltısuyu ayırım çizgisinin oluştuğu da göze çarpmaktadır. Farklı senaryolar ile yeraltısuyu çekimindeki artış sonucunda modellenen alanda yeraltısuyunda meydana gelecek değişimlerin incelenmesi amaçlanmıştır. Bu senaryolardan ilkinde; pompaj miktarında % 30 artış sonucunda akifer sisteminde meydana gelecek değişikliklerin incelenmesi, ikincisinde pompaj miktarında % 45 artış sonucunda akifer sisteminde meydana gelecek değişikliklerin incelenmesi ve üçüncüsünde pompaj miktarında % 60 artış sonucunda akifer sisteminde meydana gelecek değişikliklerin incelenmesi amaçlanmıştır. Son senaryoda; pompaj miktarında % 100 ve % 150 kat artış yapılmış ve akifer sisteminde meydana gelecek değişiklikler incelenmiştir.

Abstract:
This study was supported by the Scientific Research Projects Unit of ITU. The aim of the thesis is to conduct hydrogeological investigations for the management of groundwater and to create a groundwater flow model in the Izmit Basin where the difficult hydrogeological conditions are dominant. In other words, within the scope of hydrogeology investigation of the Izmit Basin, it was aimed to determine the critical draft amount (threshold) for possible interaction with Sapanca Lake as a result of drowdown due to uncontrolled groundwater withdrawal in the granular aquifer. Before commencing field work, a survey was made of studies in the literature previously conducted in the İzmit Basin and its near surroundings, and information was obtained on the area. In the area under examination, located in map sections G23a3, G23d2, G23b4, G23c1, G23c4, G23b3, G23c2, G23c3, G24a4, G24d1, G24d4, G24a3, G24d2, G24d3, G24b4 and G24c1, an examination had been made of the geological characteristics of the basin as part of field surveying in October 2016 and May 2017, and the geological map was revised. Digital elevation maps of the İzmit Basin have been combined using the program ARCGIS 10.3, and a digital elevation model (DEM) of the basin has been created. In order to produce a digital geological map of Kocaeli province, one section of 1/50 000 and one section of 1/100 000 were obtained from the MTA. The original data maps obtained from the MTA were joined on CBS so as to cover the whole basin, feature information was corrected, and a consistent geological database was formed. The İzmit Basin was examined for geological and tectonic characteristics under three headings: Cover Units, İstanbul Zone (North Sequence), and Armutlu-Almacık Zone (South Sequence). The units are between the Ordovician and Quaternary. Paleozoic sequences begin with the Kurtköy Formation, which consists of arkose-sandstone-siltstone alternate fluvial units. Conformably on this is the Aydos Formation, which consists of coastal-shallow marine quartz sandstone and conglomerate. Going upward in the sequence is the Gozdağ Formation, of alternating lagoon-shelf sedimentary shale-sandstone. The Permo-Triassic old unit is the Çakraz Formation, of alternating sandstone-conglomerate-mudstone. Above the Triassic sequence is the Late Cretaceous old settling, starting with fragments. In this region, there is no Jurassic settling. Cretaceous sequences are the Teksen and Hereke Formations, with sandstone-shale alternation. The Late Cretaceous is represented at higher levels by the thick marl-limestone alternating Akveren Formation, formed with a limestone and clayey limestone lithology. The Eocene sequence consists of the Çaycuma Formation, with limestone-clayey limestone alternation, and the Yığılca Formation, formed from agglomerate, tufa, andesite, basalt and volcanic sandstone. On the peninsula along the coastal strip and the coastal plain are the lightly bonded fragments of the old Pliocene (the Örencik Formation) and the alluvial sediments and scree of the Quaternary. The stratigraphic units of rock on the Armutlu peninsula fall into two main groups. The first of these consists of low or relatively high degree metamorphic rocks, and the other of non-metamorphic magmatic and sedimentary rock of Cretaceous to Pliocene age located uncomformably with Cenomanian-Turonian old limestones on metamorphic rocks. In the Armutlu-Almacık zone are the tectonically related Permian-Triassic old Sultaniye Metamorphites, the Upper Jurassic-Lower Cretaceous old Keltepe Marble, and the Upper Cretaceous old Akçay Metamorphites. Above these, they are covered as angular uncomformity by the Upper Campanian-Lower Eocene old Abant Formation. Tectonic activity in the area took place at the end of the Upper Cretaceous and folded the Eocene old units. The early-middle Eocene old Çaycuma Formation and the Yığılca Formations are gradually transitional with the Abant Formation, and are angularly unconformable on other units. The İzmit Basin developed on the North Anatolian Fault system, and is one of the important active basins located in the south-east of the Marmara Region. The North Anatolian Fault consists of a single east-west fault between the Gulf of İzmit and Sapanca Lake. It is thought that the plain between the Gulf of İzmit and Sapanca Lake formed as an effect of the North Anatolian Fault. It is seen that the fault, advancing on land and as a single fault towards Sapanca Lake, has caused displacement of up to 10 km in alluvial fans to the west of the lake. Examining the drainage characteristics of the area, it is seen that the NAF has affected streams flowing from south to north.
Making use of the long-term measurements made by the State Meteorological Stations (SMS) belonging to the Turkish State Meteorological Service (TSMS), the basin's balance of average precipitation, potential and actual evaporation and total amounts of flowing water (surface and underground) were calculated. Average amounts of precipitation for SMSs in and around the study area were calculated as follows: Karamürsel SMS (9 years), Körfez SMS (14 years), Sakarya SMS (40 years), Kocaeli SMS (40 years), Geyve SMS (35 years), and Sapanca SMS (37 years). For İzmit Basin, average precipitation was calculated using the ARCGIS program and the isohyet method. In this way, the mean precipitation of the 1370 km2 İzmit Basin was calculated. In addition to the six state meteorology stations, five İSAŞ meteorology stations which were thought to represent the basin also contributed to the calculation of average evaporation in the İzmit Basin. In this way, the average evaporation for the İzmit Basin was calculated, using the ARCGIS program and the isoevaporation lines method. The SCS-CN method was chosen for the near-actual discrimination of surface and underground flow. SCS Curve Number was chosen as a method in which calculation of the relation of precipitation to runoff can be applied with a need for very little data. Precipitation data covering 2004-2016 from the Kocaeli and Sakarya stations and runoff data calculated by the SCS-CN method were presented together. Average annual precipitation values for the İzmit Basin were calculated for the Kocaeli Meteorological Station and for the Sakarya Meteorological Station. Using the SCS-CN method, surface runoff was calculated at the Kocaeli Meteorological Station and at the Sakarya Meteorological Station. The average surface runoff value was calculated by the SCS-CN method, and the difference was taken as percolation. Sapanca Lake is fed by streams coming from the mountains to the north and south and springs on the lake bottom, and excess water from the lake is taken by Çark stream and the Sakarya River. The average annual flow from the Çark stream regulator was calculated as 100 x106 m3/year using flow values of DSİ flow monitoring for the years 1983-2015. In the alluvium, there are many boreholes and caisson wells belonging to official institutions and private individuals. Withdrawal from wells was calculated as 35 x 106 m3/year, and discharge by sources as 52 x 106 m3/year. A difference of 133 x 106 m3/year was found between feed and discharge. Taking account of possible measurement and calculation errors, the amount of groundwater which can safely be used can be taken as 60% of the difference between feed and discharge. According to this, the amount of groundwater which can safely be used in the İzmit Basin is 80 x 106 m3/year.
During the course of field work, formations in the basin were examined hydrogeologically to show rock-water interaction and the hydrogeological characteristics of units. Characteristics such as temperature (T), pH and electrical conductivity (EC) of water samples were measured in situ because they do not keep their physical characteristics for long. A thermometer, a pH electrode, and a conductivity electrode measuring salinity were used for the measurements. These instruments used to measure the physical characteristics of the samples were checked and calibrated with distilled water before each use. The water samples were stored and taken to the laboratory for chemical analysis in suitable conditions. For this purpose, when the samples were taken, 1000ml polyethylene bottles with locking caps were used. The bottles were shaken at least twice with sample water, and were filled so as to leave no remaining air. Laboratory studies comprised analysis of the anion-cation ions of the water samples, pollution parameters and heavy metal content. In October 2016 and May 2017, 41 samples were taken in the Kocaeli Basin for the dry season and 35 for the rainy season at locations determined after detailed field work, and which were thought to represent the basin. These 76 water samples were subjected to chemical analysis at the ACME Laboratories, and NO3 and SO4 analyses were performed at the Hydrogeological Laboratory of Istanbul Technical University. In order to determine the lithology which was effective in the chemistry of the water, to classify it hydrogeochemically, and to evaluate it as irrigation water, various diagrams were prepared with the program Rockware AqQA 1.2 using the results of chemical analysis. In order to determine the quality of the water according to chemical characteristics, a Schoeller diagram, a Piper diagram, a Durov diagram, a Gibbs diagram and a Schoeller semi-logarithmic diagram were prepared. The most important sources of pollution in the Izmit Basin are intensive industrial and farming activity and animal rearing. Trace element analyses (As, Cr, Cu, Fe, Mn, Ni, Zn) were also performed by Acme (Canada) Laboratories to determine pollution of groundwater on the plain. The results of heavy metal analysis showed that the maximum limit values permitted by Turkish Drinking Water (TSE 266) and WHO standards were not exceeded.
In order to create maps of the sensitivity of groundwater to pollution, the DRASTIC model, developed for the United States Environmental Protection Agency (USEPA), was applied to the İzmit Basin. The DRASTIC Model (DM) is a method of determining the geological, hydrological and hydrogeological parameters of Depth to water, net Recharge,  Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity, and mapping sensitive areas with the use of ArcGIS techniques. The variables D, R, A, S, T, I and C represent the parameters the susceptibility of the groundwater to pollution. In the original DRASTIC method, weighting coefficients of 5, 4, 3, 2, 1, 5 and 3 respectively were given. The DRASTIC Index (DI) value was calculated using all parameters in classifying and weighting. As a result, it was seen that the calculated DI value varied between 29 and 184. As a result of hydrogeochemical analysis performed in the basin, it was found that the groundwater conformed to drinking water standards. However, in groundwater samples taken from alluvial environments, a relatively high nitrate concentration was found. In the İzmit Basin, there are inhabited areas, agricultural activities and fish production operations which threaten of surface and underground water. Thus, a nitrate concentration of over 10 mg/l (13.45 mg/l) was measured in water samples taken in May 2017 at representative points in the basin, indicating anthropogenic pollution. These findings show that it is necessary to carefully protect carbonate environments which have a high potential for pollution but are as yet unpolluted, and alluvial environments which have signs of pollution. Planning and development of the use of water sources in catchment basins in Turkey is conducted by the DSİ General Directorate. In regulations concerning the protection of water obtained now or in the future for drinking or other purposes, Absolute, Short Distance, Medium Distance and Long Distance Protection Areas are defined. However, it is not possible when protecting water sources to separately assess surface and underground waters, as they are in continuous interaction. Moreover, despite work to protect bodies of surface water which provide mostly drinking water, it is seen that this is inadequate. The areas where groundwater was most sensitive to pollution were clearly found to be alluvium, old alluvial fans, alluvial fans, the Örencik Formation, and the marbles and limestones in the south. Springs in the south of the basin feed streams which reach the lake. Therefore, protection of the areas feeding these streams is important for the lake. In this situation, it is understood that in drinking water basins the current traditional protection zones defined by the maximum water level of the lakes and the protection zones of the streams feeding the lakes are inadequate for the protection of water quality. In addition to the current protection zones, "Hydrogeologically Based Protection Areas" including the areas feeding the streams emptying into Sapanca Lake and the areas above those streams which feed their base flow are recommended.
It is seen that areas which are sensitive or very sensitive to pollution are, respectively, alluvial environments and areas covered with karstic limestone. The validity of the sensitivity map is confirmed by nitrate concentration measured in the alluvium environment. Therefore, limestones and marbles and the alluvial environment which have streams directly draining into Sapanca Lake and which have not developed natural purification processes are classified as a Hydrogeologically Based Essential Protection Area. Old alluvial fan, alluvial fan and the Örencik Formation are not observed to have direct source drainage and do not have natural purifying ability, and so they are defined as a "Hydrogeologically Based First Degree Protetion Area".
After completing İzmit Basin hydrogeological studies (a conceptual geo-hydrogeological model, groundwater potential and quality), modelling work was commenced. Research findings were gathered on a database which was necessary for digital modelling. A 'Groundwater Flow model' which was necessary for preparing the optimum groundwater management plan for the basin was developed with studies conducted at the DELTARES Institute in the Netherlands. A groundwater flow model was developed making a digital solution by the three-dimensional finite difference method, with the aim of calculating the amount of groundwater and thereby developing the optimum management plan for the basin. Modelling work was carried out in two stages. In order to determine the groundwater flow of the İzmit Basin, the IMOD, based on the MODFLOW 2000 program, was used. For this program, geological and hydrogeological data, precipitation and evaporation quantities, aquifer feed quantities, groundwater levels in observation wells, and the amounts drawn from farm and private wells were used. After determining limit conditions according to the geological and hydrogeological parameters of the model area, grids were formed with IMOD, and the layer types, numbers and parameters necessary for the model were entered. Using IMOD, groundwater flow model studies were conducted in the aquifer, which has an area of 195 km2 in the İzmit Basin. A grid model of the aquifer was formed by dividing the model area into 238 000, 100 m x 100 m cells in six layers, 700 columns and 340 lines. In the model study, the lithological information from the borehole logs kept by the DSİ was used and the prepared geological cross sections were adhered to. A solid model of the model area was created with the help of the lithological information from the boreholes. For the limiting conditions of the aquifer system in the groundwater flow model, Sapanca Lake (30 m) located to the east of the model area and the Gulf of İzmit (0m) to the west were defined as constant head boundary condition (CH). A total of 200 x106 m3/year was calculated for the average precipitation (1976-2017) of the model area using the ARCGIS 10.3 program and the isohyet method. According to this, the average precipitation for the approximately 195 km2 model area was determined as 1028 mm/year (2.8 mm/day). After subtracting evaporation and surface flow from precipitation, net recharge was calculated as 306.6 mm/year (0.84 mm/day). Also, in places where clay was seen on the surface, recharge was taken as 0.20 mm/day in the model. The alluvium aquifer is fed by springs draining from the limestones and marbles in the south. The total flow from the sources in the south was calculated as 43 200 m3/day. The recharge value over six layers and 217 cells in the model was calculated as (43200 m3/day) / (6*217) =33 m3/day for each cell. A pumping or injection well is defined in the Well packet of the program. Values defined in this packet are water drawn from wells and injection. (Q, m3/day) If it is considered that the wells in the modelled area work 24 hours a day for 365 days, it is calculated that they take 97 000 m3/day of water. The model was calibrated using the groundwater level values measured in the observation wells in the model area. For detailed calibration, values such as groundwater levels and spring and groundwater drainage calculated by the model were compared with values observed in the field. Examining flow direction, a clear groundwater flow was observed to the Sea of Marmara in the west of the Gulf of İzmit, and towards Lake Sapanca in the east. It is noticeable that according to the results of the model, there is a groundwater separation line between the Gulf of İzmit and Sapanca Lake. In order to estimate groundwater levels in the İzmit Basin under current conditions until 2055, a three-dimensional groundwater flow model was developed. When the difference between the groundwater levels measured in the rainy and dry periods between the years 2016 and 2017 and the calculated levels reached an acceptable level (R2>80%), calibration of the model which was prepared was counted as complete. It was aimed to examine changes in groundwater occurring in the modelled area as a result of an increase in the taking of groundwater by different scenarios. The aim of the first of these scenarios was to examine changes which would occur in the aquifer system as a result of a 30% increase in pumping, the second the result of an increase of 45% in pumping, and the third the result of a 60% increase. In the last scenario, pumping was increased by 100% and 150%, and the changes which would take place in the aquifer were examined.