Multiple Database Connections - Multi-task c# executable

Question

I have an executable that I need to run a database giant. How the processing time would be unviable if I did it in a single Thread.... Following the idea of this question, I decided to create an executable that does N tasks in parallel... so basically I always do the same thing for different time intervals.

static void Main(string[] args)
{
        var dataInicio = DateTime.Now.AddDays(-35);
        var dataFim = DateTime.Now;

        var listaTasks = new List<Task>();

        while (dataInicio > dataFim ? dataInicio >= dataFim : dataInicio <= dataFim)
        {
            var d = dataInicio.Date;
            var meuBll = new MeuBll();
            listaTask.Add(Task.Run(() =>
                                        {
                                           meuBll.Foo(d);
                                        }).ContinueWith(
                                            x => meuBll.Dispose())
                               .ContinueWith(x => GC.Collect()));

            var meuBllNovo = new MeuBll();
            listaTask.Add(Task.Run(() =>
                                        {
                                            meuBllNovo.Boo(d);
                                        }).ContinueWith(
                                            x =>
                                            meuBllNovo.Dispose())
                               .ContinueWith(x => GC.Collect()));

            if (listaTask.Count >= 2 * 5)
            {
                Task.WaitAll(listaTask.ToArray());
                listaTask.Clear();
            }
            dataInicio = dataInicio > dataFim ? dataInicio.AddDays(-1) : dataInicio.AddDays(1);
        }
        Task.WaitAll(listaTask.ToArray());

So basically I chose to run, for example, a 10-day interval at once, as you can see in if (listaTask.Count >= 2 * 5), the problem is that my methods Foo and Boo has several connections to a single oracle database...

public class MeuBll : IDisposable 
{
    private readonly OracleDal _oracleDal = new OracleDal();

    public void Foo(DateTime data)
    {

        var t1 = Task.Run(() =>
                     {
                         _listaSucesso = _oracleDal.ObterListaSucesso();
                     });

        var t2 =
            Task.Run(() =>
                         {
                             listaFalhas = _oracleDal.ObterListaFalhas();
                         });

        t1.Wait();
        t2.Wait();

        foreach (var sucesso in _listaSucesso)
        {
            //uma logica para inserir objetos em uma "lista de merge"
        }

        if (listaDeMerge.Any())
            Task.Run(() => _oracleDal.MergearLista(listaDeMerge)).Wait();
    }

    public void Dispose()
    {
        if (_hash != null)
            _hash.Clear();
        _hash = null;

    }
}

And my method of merging:

    public void MergearLista(List<MyObject> listaMerge)
    {
        using (var conn = new OracleConnection(Connection.ConnectionString))
        {
            if (conn.State != ConnectionState.Open)
                conn.Open();
            using (var oCommand = conn.CreateCommand())
            {
                oCommand.CommandType = CommandType.Text;
                oCommand.CommandText = string.Format(@"
                MERGE INTO MyTable dgn 
                USING (select id from another_table where field = :xpe) d ON ( TO_CHAR(dateHappen, 'DDMMYYYY') = {0} and id = :xId) WHEN MATCHED THEN 
                    UPDATE SET OK = :xOk, dateHappen = SYSDATE
                WHEN NOT MATCHED THEN 
                    INSERT (fields....) 
                    VALUES (values...)");
                oCommand.BindByName = true;
                oCommand.ArrayBindCount = listaMerge.Count;



                oCommand.Parameters.Add(":xId", OracleDbType.Int32,
                                        listaMerge.Select(c => Convert.ToInt32(c.Id)).ToArray(), ParameterDirection.Input);

                oCommand.Parameters.Add(":xPe", OracleDbType.Varchar2,
                                        listaMerge.Select(c => Convert.ToString(c.Xpe)).ToArray(), ParameterDirection.Input);


                oCommand.ExecuteNonQuery();
            }
        }
    }

The problem is that for each processing day, the process takes around 2 hours... and daily has a routine to back up the bank, which makes it out for about 10 minutes, which will make my executable launch some Exception or simply lockar some table..

What do I do? I stop the executable "in the hand" and put to rotate again taking the days that he has already executed... Also, many connections are simply open... so I kill these sessions at hand too...

How do I fix this? Is there a way to force all open connections to simply close? Force Dispose theirs?